Premade masteries or choose on the fly?

Many pros/high elo players adjust their masteries every game based on individual matchups in that game so that they get the most beneficial stats they can from masteries. For example, a mid laner may have a generic mid lane mastery page in which they take 2 points in Meditation (mana regen) and 3 points in Merciless (bonus damage to champions below 40% hp). This may be perfectly fine for someone like Ahri, but if you are instead playing Zed, it would be better to take the two points in Meditation and put them into Merciless, as Zed does not benefit from mana regen.

Also, in the case of marksmen, some marksmen do well with thunderlord's decree, while others like Jhin benefit more from Deathfire Touch. Also, if you are in a matchup against a heavy poke lane, it may be better to take Wardlord's Bloodlust to avoid being pushed out of lane as easily.

TL:DR It is fine to have premade mastery pages to save time when setting them up, but if you want to optimize you stats gained from them, you should always try and fine tune them each match based on the champions you are facing.

Adapting for each game is better, but not necessary. Similar to runes, your masteries help affect your early game, which can determine the state of the game.

Adapting them to your enemies gives you the benefits of more tanky stats, damage or utility depending on your matchup or team composition. For example, if you are against an opponent that you can trade or harass a lot, choosing Thunderlord's may be good as it gives you more damage in the exchange. On the other end of that trade, if you have a shield or heal ability, Windspeaker's Blessing can help you recover health or mitigate damage.

If you are interested in what others use, you could check out Champion.GG. There you can look at specific champions, most taken masteries, masteries with highest win rate, and item builds.