How many crew do I really need?
tl;dr
always* go with a full house**, keep enough spare Rations, and be Unaccountably Peckish.
*unless
a) you already have Eschatologue, >100 skills, and some game experience - by then, if you already did all the most crew-risky stories, you can probably safely keep the crew to 32 people to have both a nice spare crew buffer and low-ish Hunger increase at the same time,
b) you're broke or almost broke (for the obvious reasons),
c) you've decided you're going South to explore the Elder Continent; since you're getting your crew reduced to either 1 or 0, the starting amount can be safely kept as low as possible smirks. this is the one situation where Steam Launch does come in handy chuckles.
**or full house minus one, if you wish to benefit from (rather rare though) +1 zailor random events
To reiterate: you need 1/2 of the max crew of a given ship to be able to keep you engine at regular max (2x) speed (so yeah, the number goes up for bigger ships, as would common logic suggest) - having ship speed at 1x means 2x time used, 2x food consumption, 2x terror, 2x zee-risks etc.; still, if you're a serious player, you'll want to have a full house. The more the merrier, yeah? It's not just an opinion though; sadly, the other answers are wrong, because they have overlooked a couple of quite important factors. I'll explain them one by one. Go science!
Firstly, increasing your crew doesn't necessarily increase your food consumption! Hunger is quantized (as you can easily verify on http://sunlesssea.gamepedia.com/Hunger), so unless you're just beginning your game (and have a 10 crew sized ship), there ain't no Supply upside to having e.g. 19 crew instead of 24 - both will (in 99% of the situations) chew through your biscuits at exactly the same speed! In reality, you're likely to see an impactful difference only on the smallest (10 crew, 40 cargo-ish) ships. Even in those starting cases, you should never aim for "1/2+1" (e.g. 6 crew on your starting ship), for the reasons described further below.
Secondly, crew is cheap. You can easily get 1 crew for 5 to 15 E in London (depending on luck and circumstances) - and that means just pocket change for fully staffing even the biggest ships. Hell, crew is usually cheaper than rations (unless you're in Naples, that is), so you're still usually much better off with 30 crew and 0 rations than with 16 crew and 5 rations, Hunger-wise. I won't provide the grim details here, but I strongly suggest getting Unaccountably Peckish ASAP to any serious Zee-Captain. While feasting, don't use Mouths to Feed (it is 1.5x more efficient zailor-wise, but about 2x less efficient Terror-wise) - wait for STARVATION WALKS THE SHIP!. Note that paying the (quite slight IMO) +5 Terror cost gives you -50 Hunger (half the bar) for one crew "donation" (1-2 zailors) on that event, as opposed to -20 Hunger for +1 Terror choice there. If you keep your Terror low (<50) most of the time and thus have some free Terror to spare, it does make a difference (for 30 crew ship) of about 5 vs 2 full bars fed, that is 10 vs 4 Rations, respectfully.
Zee is unforgiving, my dear mates.
Thirdly, if you're a beginner, it's extremely easy to lose crew (mostly because it takes a lot of play time to know where you'll lose crew and to be able to avoid it) - OTOH, if you're experienced, you're probably Merciless, so it's also extremely easy to lose crew (yup, there's a pattern here smirks) - and the consequences are very dire. While you probably won't have 0 crew as a result of those encounters, getting below 1/2 crew is an almost instant death in most of those situations.
As to those "sticky situations" - high terror triggers crew death events; many random events trigger multiple crew loss on failure (some even on success) - creature attacks on less than 1/2 hull triggers crew death (often more than 1 for larger beasts). Imagine you're fighting a zee-beast with the "1/2+1" crew pattern described in other answer. Let's even say it's a smaller beast or a small vessel, with only -1 crew attack (if it's a -2 or bigger, you're screwed already). You got hit once too many (got to about 1/2 hull) so you decide to show that discretion is the better part of valour. You try to escape, hit full throttle, but the beast charges (most of them do) and is lucky enough to score a hit, throwing you at the shore. You just got to 1/2 crew, but theoretically you can still escape - well, not in this case, because before you can turn your ship and get out of the beast's range, it charges again - this time reducing your crew to 1/2-1, effectively making it impossible for you to escape even on full throttle. Hope you've written your Will, mate.
As to the story encounters - as you probably guess, you can easily lose e.g. 2 crew in them - and then you'll probably have to go half-speed all the way to London (or recruit in Khanate for the outrageous prices they have), making you not only lose many more rations that those spare crew would eat, gain a lot of unnecessary Terror and turn you into a sitting duck for Zee-monsters - it also wastes a lot of your RL time. I hope that makes my point here.
Fourthly, since crew size is usually proportional to both the cargo capacity and the amount of Echoes you can earn on trades or recurrent events, the Supply downside is mostly negligible. Let's define a quality, crew size in Supplies (e.g., for 10 crew it's just 10 Supplies). Just carry about 1.5x csiS for your regular travelling runs, about 1x csiS for cargo runs or travelling runs along Supply-rich ports, and about 0.5x csiS for cargo runs along Supply-rich ports. If you Supplies drop below that value, restock ASAP before spending Echoes on anything other than Fuel (i.e. Fuel still has priority). You'll never have to worry about Supplies again (well, at least not until you forget to restock or are extremely unlucky, which is quite common on the Zee chuckles).
Fifthly, excess crew is just Nice. Not only you can take more risks (e.g. you can go down to about 1/3 max Hull while fighting and don't mind the crew loss at all, you can trigger those potentially high-gain, highly-crew-lossy events etc.), but you can also just go to Naples with them. Bear in mind that hauling cargo there always costs you 1 to 3 crew, but, at the same time, gives you an average net income of about 200 Echo and 30 Fragments per run (crew recruitment cost included). As such, if the Canal is along your route, you can just drop the spares there.
So, to sum up, always* recruit up to max when you're in London. You won't regret it (in worst case, they'll regret it chuckles).
also, FWIW, I find the WE ARE CLAY a bit lacking; benefits of -3 to crew and +5 Iron with a very minor Engine Power gain is IMO hardly worth it to waste a ship's Auxiliary (+7 to any stat, Pages probably, or +10 to all with Zong of the Zee) slot alongside losing one more possible cargo run on Naples and increasing the risk of crew deficit. YMMV, though; it certainly doesn't hurt much to have them stored just in case of emergency in your cargo hold.
If you upgrade to a ship that requires 25 crew, you'll need at least 13 to be able to use the 2nd level speed. You might consider grabbing the WE ARE CLAY from Polythreme, which reduces crew quarters by 3. So if you have a ship that needs 25 crew, add WE ARE CLAY, it means it takes 22 crew, and you only need to run 11 members.
I tend to run with 1/2 crew + 1 (because there are random events which can kill a crew member, and if that happens all the way across the zee, limping back to London is no fun at all).