with standard OML/RSL multiplexing :

1 E1 has 32 timeslots, with:

1 TS for physical signalling/synchro

1 TS per group of 4 RSL (=4TRX)

2 TS per TRX

a BTS with 4 TRXs use 10 E1 TS (1×4)

a BTS with 5 TRXs use 13 E1 TS

a BTS with 6 TRXs use 15 E1 TS

a BTS with 7 TRXs use 17 E1 TS

a BTS with 8 TRXs use 19 E1 TS (2×4)

a BTS with 9 TRXs use 22 E1 TS

a BTS with 10 TRXs use 24 E1 TS

a BTS with 11 TRXs use 26 E1 TS

a BTS with 12 TRXs use 28 E1 TS (3×4)

a BTS with 13 TRXs use 31 E1 TS

a BTS with 14 TRXs use 32 E1 TS (4/4/6)

In Alcatel, with a more agressive multiplexing, one can reach more TRXs per E1 (but less radio timeslots for TCH can be defined) (up to 16 TRX / E1). But this is not recommended, because nobody wants to loose TCH capacity 🙂 It can be useful for remote BTS with Abis over satellite.