For the most part, the AP brand doesn't matter. It is all about the channel. An 11g channel has about 20 Mbps of possible throughput, but that is only in a very clean RF environment.
In a more realistic environment, it will go down to 15Mbps quickly, and even lower if there are any 11b devices in the area.
The number you will hear floating around is about 20-25 devices per non-overlapping channel if they are just surfing the Internet. That is just about the maximum and I don't recommend trying to push it beyond those numbers.