How do I determine the optimal maximum simultaneous transfers in Filezilla?

For this, there is no right or wrong answer.

For me personally, I like setting this to 10.

However, you have to remember that many public/private servers have limits either on the total speed of individual connections, speed per ip address or amount of connection.

If they have total speed, quite frankly setting to 10 will only help in situations where you have many files (as it can save time on the authenticating step.

I just transferred 3235 files from my web server to local - and, whilst every file is very small, and the server has a gigabit link, there is about .5-1 second of authenticating and setting up the connection for each and every file which you just can't help when using FTP. By setting 10 files, I greatly increase the overall time of downloading all the files.

In addition, the reason I like setting this to 10 is because, when you connect to some servers, it is possible (due to many factors) to start a transfer at a very low speed (e.g. the server may be under load), and the more connections you have open, the higher the chance of utilising higher speeds, due to the fact that they may be equally sharing the speed between connections (which is quite common).

With the exception of a couple of sites that do have transfer limits, I personally set this to 10 on my machine and have for many years. I recommend you do the same - and check the rules of the server you are connecting to as they may have rules. (or no rules, but, just limit you anyway!).

Note that if you are only transferring a few (smallish) files, this really will not make much of a difference at all.

As for I/O, again, this is a hint to try and get a bit more out of the target server. It is highly possible to get the same speed from a single connection as with 10. Windows has a great read/write cache ability and I highly doubt that this will be a bottleneck. If you do want to test, you can always fire up Process Explorer and look at the I/O graph to see if it is anywhere near max.