I think the starting point would be to establish the paging capacity of your Lacs. The evaluate the paging load at BH and size your lacs accordingly.
To get the paging capacity consider that in one hour you have a maximum of 3600 / 0.235 *(9 – BS_AG_BLKS_RES) *pag_msg_per_pag_chn *eff% in uncombined BCCH. replace 9 with 3 for combined in the above formula.
3600/0.235 is the max ccch blocks you may have in an hour given the duration of the tdma frame is 235 mS
9 – BS_AG_BLKS_RES is the total PCHs you get from the radio TS when uncombined
pag_msg_per_pag_chn is the paging channels per paging message depending on the paging mode implemented ie imsi-imsi, tmsi-tmsi etc. Usually there are either 2 or 3 paging messages per paging channels
eff% is the efficiency level to which you wish to load your paging channels. 80% is a safe limit if you can handle the signalling traffic that results from redifining many LACs
Compare these capacity thresholds against the BH paging messages as recorded by the omcr counters and make your decision on the size and number of LACs you need