Rental pets??? u mean the ones u can get from the Pet Npc...
well if is that what u are meaning, it is easy, simply change the checkmarry in the npc... example:
This is taken from my npc script... simply change the checkmarry from 5 (which is 5 mobs) to the number desidered:
#IF
checkmarry 5
checkjob Wizard
#ACT
marry mob KekTal
take Gold 1000
#SAY
Peter:"Here is your pet take care of it"\ \
<I need more/@buybaby>\ \
<Exit/@exit>
#ELSESAY
You have reached 5 mobs or you dont have money\ \
<Exit/@exit>