Is it 64 or will a reduced number be more efficient?
Typically, a VPS provider will throttle or kill processes which eat up too much CPU.
That said, each mining thread will need 2 MB of fast (cache) memory. So you'll want to have one thread per 2 MB of cache you have, even if you have more cores available. You will want to play around with different values around this to see how your particular system behaves though, as other processes may jostle for cache usage too.