[FW1] Licensing issue

[Systems Admin]

Hi all, I got 14 nos. of static IP address allotted by the ISP. They are allotted to webserver, SMTP, ftp, dns, proxy and etc. I got more than 100 users accessing the internet covered by the proxy. I presume these 100 users are represented at the firewall as one single IP address. My question is how many user license for the FW-1 should I buy?   Any helpers please?   regards suresh