It's not related to how many site-to-site tunnels you want to build.A license for single VPN-1 UTM gateway would be enough.You can build as much site-to-site tunnels you want If you want to use secureclient IPSEC VPN then its related to the amount of users. Check
http://pricelist.checkpoint.com/newPL/index.asp.
Send a mail to CP, they will tell you exactly how it works with the licenses.