Description: "...in a range from a to b inclusive."

You should explicitly mention that a is exclusive, otherwise someone else will wonder why one of the last tests does not pass

Don't you see the 9 in exponent?

Done

Still, not fixed.

Should the instructions read 3 < n < 10**9? (not 3 < n < 109)

9 + 9 ?

Hello! I think the C# example test cases are incorrect:
Assert.AreEqual(2, SumOfSquares.NSquaresFor(17));
Assert.AreEqual(2, SumOfSquares.NSquaresFor(18));
Assert.AreEqual(3, SumOfSquares.NSquaresFor(19));
How can this result be possible for 18 and 19: 18 = 16 + 1 + 1, 19 = 16 + 1 + 1 + 1. So it must be 3 and 4.
Or maybe I just dont understand something.

Random test cases are susceptible to cheating.