How many unique primes?
MATL, 4 3 bytes

-1 byte thanks to Luis Mendo

YFz


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YF         Exponents of prime factors
  z        Number of nonzeros


Original answer:

Yfun


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A verYfun answer.

          (Implicit input)
Yf         Prime factorization
  u        Unique
   n       Numel
           (Implicit output)