What is the maximum number of ipv6 addresses that can be assigned to one ipv6 interface?

Sample run, to give you an idea why that is:

R1#show ipv6 int

FastEthernet0/0 is up, line protocol is up

  IPv6 is enabled, link-local address is FE80::200:11FF:FE11:1111

  No Virtual link-local address(es):

  Global unicast address(es):

    2001::1, subnet is 2001::/64

    2001::2, subnet is 2001::/64

    2001::3, subnet is 2001::/64

    2001::4, subnet is 2001::/64

    2001::5, subnet is 2001::/64

    2001::6, subnet is 2001::/64

    2001::7, subnet is 2001::/64

    2001::8, subnet is 2001::/64

    2001::9, subnet is 2001::/64

    2001::10, subnet is 2001::/64

    2001::11, subnet is 2001::/64

    2001::12, subnet is 2001::/64

    2001::13, subnet is 2001::/64

    2001::14, subnet is 2001::/64

    2001::15, subnet is 2001::/64

    2001::16, subnet is 2001::/64

    2001::17, subnet is 2001::/64

    2001::18, subnet is 2001::/64

    2001::19, subnet is 2001::/64

    2001::20, subnet is 2001::/64

    :

   2001::200, subnet is 2001::/64

answer; (& & );

answer ingress;