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
...
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
...
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