Hi machzar,
It is quite common to use multiple LACs under one BSC. However, we can have several LACs, as long as number of BTSes under each LAC is not huge. I remember having something like 8-10 LACs under a BSC but around 6 of which had less than 5 BTSes attached to it. So again, its just a question of loading the BSC. I am talking about Ericsson BSC by the way.
Cheers,
Aladdin.