The radio system in some wireless bases is indeed a PC card, while in others it's discrete chips on a board. The card used is up to the equipment designer-Linksys uses one vendor's innards, while Apple uses another, D-Link uses still another, and so on.
The max number of connections is based on how many separate MAC addresses the base station will be able to keep track of. Not really a 'physical' restriction, but a restriction none the less. You will probably see some degradation in throughput before you reach the max connection limit, simply because of the amount of data going back and forth.