In case of high load, the number of PDCH will decrease anyway, to give room for TCH.
The only rule I can think of:
MAX PDCH < number of TS on all your TRX which have TRX PREF MARK = 0
MAX PDCH < total number of TS in the cell
MAX PDCH < total number of TS in the outer zone of the cell (in case of concentric cell)