10. is /8 (24 payload bits), 172.16 is /12 (so 22) and 192.168 is /16. Very little need to spend more than 18 bits of space to map every 'usable' private IPv4 address once per customer. Probably also less than 14 bits (16k) of customers to service.
There's more addresses I didn't know about offhand but found when looking up the 'no DHCP server' autoconf IP address range (Link Local IPv4).