1/ usually TMSI, except if MS wasn’t allocated a TMSI in the first place or if the NSS parameters force the paging to be done on IMSI, or if this is a repetition of the paging and due to NSS parameters, this repetition ought to be done with IMSI. Those are quite special cases, it seems, so I assume that 90% of paging are based on TMSI.
NSS guru, please confirm (or infirm…)
2/ paging capacity on which interface ?
Air interface supports usually between 50 to 80 paging/s (@100% load), depending on the parameter AG_BLKS_RES, and with 9 CCCH (= no m-CCCH)
Other interfaces and other equipments have a higher paging capacity, so the limiting factor is (usually) the air interface.
the second limiting factor is the BSC paging capacity, so if you put too many LAC per BSC, that could be a problem. This is vendor dependent.