“I want to ask how many sessions in total can the system support? is it just a few sessions?”
Sessions=you mean number of calls?
it depends on the capacity of your server.. see the specs of your server so that you can see the maximum concurrent calls it can handle.. Many Servers can let you make many extensions, some actually are unlimited, but calling at the same time is limited to its capacity.. some can give you 4 concurrent calls, others more than..