There are normally upto 1900 rooms on MX although getting that list on Primary connection can be difficult sometimes.. its not because of MX its because of usually a firewall.. They see the UDP flood that comes in as a threat and block it. So the fix is
As Max says:
Start MX as a secondary and get a list, then on the netowrks page switch to primary and run MX.
Turn off the firewall while the list loads and then turn it back on after its finished.
See if those help you out...