Be careful on the words you use 🙂 BSS is the whole system, from TC to BTS. One BSS is your whole “radio” network.
In one BTS, you can have one cell, up to six or eight cells. Usually, in standard configuration, a BTS carries 3 cells of the same band, or 6 cells (3 cells GSM900 and 3 cells DCS1800).
In one cell, there is a certain amount of TRE’s (transceiver equipment). Each TRE can carry (in average) 1 TS for signalling and 7 TS for traffic (packet or voice). On 1 of those cell’s TRE, there is one timeslot dedicated for broadcasting (BCCH).