The question is a bit unclear. You can make as many namespaces as you like. So in any given compilation unit there could potentially be hundreds or more namespaces (although that would be uncommon)
Officially from the standardization committee, there is std. There is also std::tr1, although I suspect that that will be deprecated when the new standard is complete.
Microsoft provides stdext for a few extension libraries they provide, although there are now suitable replacements for those in std (VS2010) or std::tr1 (VS2008).
Another common one you will bump into on these forums include boost.
Most library providers will wrap their code in a namespace as well.
