I would run what I can under 1 worker process / app pool.

I don't see any reason not too.

You servers don't seem very busy. You should easily be about to cope with 65 ppl on your servers at once.

You should be about to do thousands.

Overall you will need to look at why things are slow or you. often this is the db bottleneck but look for slow running pages and optimize them.

Online tools like stackify and leansentry will help too.