Actually, I believe that the increase to 10 tunnels on the PIX501 was in the 6.3.3 release:
Increased Number of IPSec VPN Peers Supported on the PIX 501 Security Appliance
PIX Firewall Version 6.3 increases number of site-to-site and remote access VPN peers supported on the PIX 501 from 5 to 10, enabling greater VPN scalability in small office, home office (SOHO) environments.
Jayson