Omnibus phone system

We are looking to use Asterisk on a preserved railway in the UK. We have an omnibus phone system that allows signal boxes to talk to each other by pressing a digit on the handset that calls a specific phone in another box. If needed, all the phones can be connected to the same line and all boxes can talk to each other to resolve problems etc. Basically it is a direct speech channel with calling buttons.

Looking at Asterisk, a similar system could be set up by using a conference facility. Are there any limits to the number of people connected to the same conference room and could a similar system be programmed on the keys on a particular phone? Or can anyone recommend another idea?