There are actually two choices. If the ISP wants to use subnetting (a router with 32 output ports) then the prefix length for each customer is nsub = 32. However, there is no need for a router and subnetting. Each customer can be directly connected to the ISP server. In this case, the whole set of the customer can be taught of addresses in one single block with the prefix length n (the prefix length assigned to the ISP).