It's a single number in that if you take an IQ test one time you get one number, but that doesn't mean you'll get that exact number every single time you take an IQ test. Even ignoring more complex questions about them, your score on an IQ test will vary depending on simple things like how tired you are when you take it, so in practice there's some variance and you do not always get the same number every time you take a test.

This is a deeper question than it sounds. The "point" of a modern IQ test is to identify cognitive deficits to target interventions. It's abused widely among non-practitioners as a ranking of intelligence, which it is not.

I mean, nothing in the human body can be truly represented by a single number.

Even height and weight change throughout the day. People are typically taller and lighter in the morning than in the evening. Weight especially is variable, it can fluctuate up to 5 to 6 pounds.