Q: What is the coverage of the above AP (inside the building)
A: You are right. It's hard to make an approximate as internal office structure is hard to judge: file compactors, floors, partition, location of the AP's, interference from other equipments/power units, etc. These make "approximate" very difficult. The best way to get the deployment right the first time is to conduct a wireless site survey. This way, you'll now exactly how many to deploy and where to deploy.
Q: How many clients can connect to a single AP?
A: Cisco has always recommended that a maximum of 12 is recommended. If you allow a large number of clients per AP ratio, the lower the wireless bandwidth they'll get. This ratio can be found in the GUI for the 21XX, 4XXX and WiSM but I'm not sure if this is applicable with the 526 controller.
Does this information help you?