in a BCCH timeslot, there are 9 blocks, that can be allocated for AGCH or for PCH. The recommended configuration is 5 blocks for AGCH and 4 blocks for PCH.
There are usually possibilities to change this configuration, or even select a dynami configuration, depending on AGCH load or PCH load.
Finally, don’t forget that 1 PCH block can contain 2 Paging messages or 4 PAging messages, depending on your MSC configuration (IMSI paging or TMSI paging)
If you’re using “IMSI paging”, you can double your paging capacity by selecting TMSI paging.