Comment by mjevans
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).
That's all true on a statement level, but doesn't make an IPv4:IPv4 NAT solution better than either VRF/encap or IPv6 mapping.
The benefit with VRF/encap is that the IPv4 packets are unmodified.
The benefit with IPv6 mapping is that you don't need to manage IPv4:IPv4 tables and have a clear boundary of concerns & zoning.
In both cases you don't give a rat's ass which prefixes the customer uses. That math/estimation you're doing there… just entirely not needed.