the pagings are sent on the RSL, and those channels are usually defined with a variable size. So the capacity cannot be calculated (or let’s say that i’m not aware of the method).
However, the ABIS is probably never a limitation regarding paging. The limit comes from the radio PCH capacity, and also from the BSC processing. Deoending on your vendor, you might be able to find an indicator that measures the RSL load of the Abis.
(RSL is alcatel specific : it is the channel on the Abis that carries all radio signalling for 1 TRX only)
FYI, to further decrease the paging load on the Abis, you can activate a feature that aggregates multiple paging commands into one big paging message.