I have a customer that has done 16-20 in a MeetMe conference. They have done this from CCM 3.3, 4.0 and UCM 5.1. It is not recommended or supported over 6. It depends on your DSP resources. You adjust the maximum number under the Service> Service Parameter> (choose a server)> CallManager page.
You can kind of secure a MeetMe by front-ending the MeetMe number with a Unity mailbox. Then callers are announced and can be admitted or rejected at that point. But as Phoneguy69 said, it is still not really secure.