In the default mode, as far as I noticed, the Onion does NAT from the AP to the normal wifi.
This means that clients connected to the Onion get a 192.168.3.something address (as far as I remember) and not, for example, a 192.168.1.whatever address for your normal "main" wifi.
In this mode if the main wifi goes down your clients connected to the onion will still be given IP addresses and will still be able to communicate between eachother. They will just lose access to the internet and likely to DNS.
It is always possible to reconfigure this AP mode though so it works differently. But as far as I remember and noticed the default mode works as above.
I'm unsure on the maximum number of clients information.