how much of sklearn can I use with pypy?

The pypy project is currently adding support for numpy. My impression is that sklearn library is mainly based on numpy. Would I be able to use most of this library or there are other requirements that are not supported yet?

Officially, none of it. If you want to do a port, go ahead (and please report results on the mailing list), but PyPy is simply not supported because scikit-learn uses many, many parts of NumPy and SciPy as well as having a lot of C, C++ and Cython extension code.