How many IPv6 address are possible?

In theory there are 264 available network allocations in ipV6. It is not known how many are actually in use at this time.

For a very clear comparison, in IPv4 there is a total of 4,294,967,296 IP addresses. With IPv6, there is a total of 18,446,744,073,709,551,616 IP addresses in a single /64 allocation.