ITG with 2MB is almost equal to PRI2 programmed with 30 channels. You save nothing. Or you will use that 2MB also with your data networking?
Such 24 channels can let 48 people talk at a time, considering no other people talking behind every person.
Seriously, 24 channels can give 24 call connection.
How many KB? Depends which codec you use in programming your ITG. There is a good Excel table in Nortel website for its computation, unfortunately I had no more access for it.