it highly depends on the traffic you’ll carry in the BSC.
4 E1 = 123 channels + 1 SS7 timeslot
then you can use your erlang B table to find out how much channels you need to carry your traffic.
the SS7 uses usually 1 timeslot for 4 E1 (typical configuration rule, a little over-dimensioned though)
If you need 20 E1, then you’ll need 5 SS7 timeslots.