actually, 4 ports per... only 2 actual ports are used by sockets, but RTP has to be on "even" ports, "RTCP" (which is supported in later MBG versions) is on "odd" ports.
so... it goes like this.
20000 and 20001 are reserved (20000 for TNA and 20001 for tftp)
the last port is also reserved for TNA (so TNA can tell if your port range is open).
4*number of active calls. We only allocate socket when a set is in a call.
In your case, 10 sets, assume all of them could be in a call at once.
40 ports for sets. plus the reserved 4 (last port and since it's likely odd, the even one before that).
20000 - 20044
But as was pointed out above, it's not a security issue to open up more, any port that has no socket will reject any traffic to it.