MacOS version 8 or later is preferred. Macs, PCs and other computers can all share the router on the same network as each other. No - both 64Kbps channels are carried down the same cable so any equipment can use either or both channels. Although your ISDN wallbox will normally have two digital sockets, they are identical to each other and do not carry one channel each. This is all handled by Windows or MacOS automatically.
Subsequent users can share the connection.
That depends on the specific model. The router sits on your network just like another PC might - everyone is a peer to the router directly, there is no controlling computer and runs completely independently. This is generally limited by practical constraints. You will have either 64 or 128Kbps to use with your router - if two people are transferring data, they will get half of that each, and so on, so if you have lots of users, they'll each not get much bandwidth. Typically, even with five or six users surfing the net at once, they're not all downloading at the same time - some might be downloading while the others are just sitting reading a web page.