As reported in PYTHON-1762, the default of 0 is confusing. It implies that by default you get no results, or at the very least that it's possible to get no results on purpose. In reality 0 means "all results" in MongoDB. Let's change PyMongo's default to None, and change the validator to require a non-zero value (negative values are also valid, for backward compatibility with pre-3.4 versions of MongoDB) to make it clear you can't request 0 results.

This is a backward breaking change since 0 has been the default since the very first version of PyMongo. We can't make this change until 4.0.