Paging is on control channels (BCCH or CCCH). If you do not use combined BCCH you have 9 CCCH blocks (1 block = 1 paging message to 1..4 subscribers). Volume of paging is related to the Location area size. Check how many pagings you have per hour (140000 CCCH blocks per hour for MainBCCH); how many blocks you have reserved for access grant channel.
Also check if you use IMSI or TMSI (2 TMSI = 1 IMSI)… and paging groups (multiframes between paging) smaller value -> earlier paging congestion.