Hi,
on PIX after specifying NAT, there is an option to specify max_conn, if you sopecify 0 there then it mean unlimited connections can go through that pix.
on PIX, you can use PAT. With PAT you can use 64000 hosts to access the same IP. In this scenario you can use PAT :
you can give the command
global (outside) 1 interface
and make an access-list to permit that external hosts to come inside the network and apply that ACL to "in" direction of outside interface.
kindly update for further queries.
regards
aashish C