I am a strong believer in the fact that everything depends on your environment. If you have lots of users that are constantly roaming around you want to make your deployment as dense as possible. I would try not to have more than 25-30 clients at max, and realistically for best performance I try to keep a high water mark of 15-20 clients / AP.
Do not add any more than 3 SSIDs otherwise you will degrade performance.
In school environments you will find that users tend to migrate to places that you had no idea they would be using their devices :) so be prepared to do some tuning when you get things fired up.
Also if you are thinking about BYOD this adds to the mix, so instead of one device per user, you could be looking at two or three, it is important to not base your coverage on your users, but expected devices.
Obviously this is not technical, but my personal expeirence with the AP105.