It makes sense to think of Bitcoin as a substance, like gold. I.e. uncountably. That's why, when talking about amounts, I don't use it as a countable noun; it's "how much bitcoin", not "how many bitcoins". You don't ask "how many golds does this earring have?", do you? (Not to be confused with capitalized Bitcoin, which is the network / tech / entity / concept.)

It's important that Bitcoin has a fixed supply, but the 21 million is irrelevant. It's an arbitrary number (in the context of its monetary properties anyway). The actual supply is 1, and ~0.92 of it has been mined already. (Note that you can't make a similar of statement about the USD. The closest approximation would be to say the dollar's supply is infinity, and 0 of it has been printed already - and only under the unrealistic assumption it will live forever. Doesn't sound enticing, huh?)

The smallest unit we use - currently sats - is just a parameter for a mapping from the continuous range [0, 1] to a discrete range of integers 0..n-1 so amounts can be represented more conveniently, in orders of magnitude we're used to. A quantization; like converting an analog image to a bitmap. As the value of the smallest unit grows too large to represent the value of the things we buy (we zoom in and our prices "pixelate") we just increase the resolution and introduce millisats, nanosats etc.

Some people have this bizarre idea that the fact we can increase the resolution of our quantization means Bitcoin is not scarce. That's like saying 50.648 is 3 orders of magnitude greater than 50, because it has 5 digits and not 2.