Cisco's rule-of-thumb is 25 wireless client per AP. But this number can go up or down depending on the type of traffic. If, for example, you're going to say that traffic will be mostly HTTP/HTTPS, emails, IM and no video or RTP streaming then you can get away with 35 wireless client per AP. If you want to do HD video streaming then the number drops to about 8 wireless clients per AP.
Ok, now the easy part is finish, here's the hard part. What's your budget like? You have several choices. You can either go autonomous IOS (this means you've got to configure each and every one of the AP individually and dynamic channel assignments is turned off and dynamic radio power control is also turned off) and this will be a very cheap-and-nasty solution.
You can also go controller-based solution where you configure the controller once and the APs will pull their configuration from the controller. This solution gives you the added benefit of a unique Cisco feature called CleanAir, dynamic channel assignment and dynamic radio power control.
