Concurrent Calls
Concurrent calls are the number of calls you need to and from the outside world at the same time. This has no bearing on extension to extension calls. You can have as many extension to extension calls as you like.
Extensions
Extensions are the endpoints you will use to make and receive calls, they may be regular desk phones, softphones, door entry devices and more.
Incoming Numbers
Incoming numbers are regular numbers that people from outside can reach you on, you can have one for the entire company or one for each extension or anything in between.