Does anybody know if the PIX 501 10 User license will restrict how many users can traverse a site to site VPN that terminates at the PIX?
Yes, it does, I've run into an issue with it myself in the past. Per the product page at http://www.cisco.com/en/US/customer/products/hw/vpndevc/ps2030/products_data_sheet09186a0080091b18.html
it says "The Cisco PIX 501 10-user license supports up to 10 concurrent source IP addresses from your internal network to traverse through the Cisco PIX 501."
In my case what happened was that we had a site-to-site VPN established with a small office that added a few more employees, everything was fine until the 11th IP address attempted to connect to a resource on the other side of the IPSec tunnel. We resolved the issue by upgrading to a 50 user license.