A perfect square number will always have a prime factorisation, with _____ number of prime factors.

A perfect square number is the product of a number multiplied by itself. 

For example, we can write 81 as

$81 = 9 \times 9$

or, $81 = 3\times 3\times 3\times 3$

We can see that the number of factors of 81 is even in this case.