No, Ahey does not support password-protected or private channels. However, you can use a unique or hard-to-guess channel name - for example:
81df86c0-6fb5-4bf3-b6e8-14f5d530edad
This makes it highly unlikely that someone will stumble upon your call accidentally.
There is no enforced upper limit on the number of participants in a channel. However, Ahey uses WebRTC mesh topology, which means each participant connects directly to every other participant.