One.
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If you can only make one rectangle, your number's prime. If you can make more than one, it's composite.
Given P = Prime number then you can build (P-1)/2 number of rectangles given any prime number. All prime numbers are odd and you would need two squares to make a rectangle. Given that you need two squares to build a rectangle, you divide your prime number by two to get the number of "pairs" of squares and thus the number of rectangles you can build. Since the number 2 does not divide evenly into an odd number - all prime numbers are odd - then you either subtract one from the prime number before dividing or discard the remainder if you choose not to subtract one before dividing.
81 - 7
16 - 9 = 7.
25 = 23 + 2