Hii Yash,
4+4+4 con figuration means, there are 4 TRU in each sector. One TRU has 8 time slots. Among 8 timeslots 1 is for BCCH and 2nd is for SDCCH & other 6 timeslots are for Traffic. So in one sector having 4 TRU u can make 30 calls.
i.e., For 1st TRU 6 timeslot and for 2nd TRU 8 timeslot and for 3rd TRU 8 timeslot and for 4th TRU 8 timeslot = 30 Timeslots. For 4+4+4 configuration u can make 30+30+30=90 calls.
Regards
Sambit