There is no information about the minimal number of PCH blocks in the frame, when using the AG_PREMPT_PCH = 1.
So I guess the AGCH can be assigned on all the CCCH blocks when needed, even though i doubt the case will ever happened !
(cf. PAGING AND ACCESS GRANT CONTROL B7 (or B9), p.50)
“Each PCH occurrence, the BTS will use this order to search the waiting queue for a pending message.
If there is a pending Access Grant message, and there is no PACKET DOWNLINK ASSIGNMENT in
the Paging Group’s associated queue, the BTS will preempt this PCH block to sent the Access Grant