It actually depends on what you're taking a percent of.
As much as 15% of the water was spilled.
As many as 15% of the cookies were burnt.
15% of a glass of water is a 'mass' of water - water is (basically) always uncountable. You wouldn't say 'one water', 'two waters', etc. - unless you're a waiter taking orders! But 15% of a platter of cookies is a 'number' of cookies. Cookies are countable.
That said, this smacks of the kind of distinction that the GMAT rarely tests. When it tests something like this, there's almost always another, easier, more grammatical rule being tested in the same answer choice.