Your question doesn't make a lot of sense... there's no concept of "prefixes" in IPv6 multicast. The base RFC defines an IPv6 multicast address as looking like this:
| 8 | 4 | 4 | 112 bits |
+------ -+----+----+---------------------------------------------+
|11111111|flgs|scop| group ID |
+--------+----+----+---------------------------------------------+
This means that, within each multicast scope, there are only 2^112 (5,192,296,858,534,827,628,530,496,329,220,096) multicast groups available (not many, but I'm sure you'll agree it's significantly more than 256).
There are a few "well-known" multicast addresses are available in an IANA-managed register. There are also ways to generate source-specific multicast addresses.
Of course, in proper IPv6 style, all of this is complicated somewhat with various options and flags, however all of the addressing schemes described therein provide 2^32 (about four billion) multicast group IDs per unicast /64 (typically called "a subnet" in IPv6). I think that should be enough for anyone in the nearish future.