It's an expression.
Lastly, your critique of my remark is disproved by your own words.
No it isn't.
If scientists "propose as many possible explanations as they can think of" and assuming all scientists are reasonably intelligent (a leap, I know), then every scientist must have put forward at least one theory each.
"Must" they? Unsupported (and obviously false) assertion -- Ten yard penalty.
Nowhere in "my own words" is there any implication that *everyone* is going to put forth a theory. Only that "people" are going to do so. The number of interested "people" on any given topic who are willing and able to put forth a theory is quite small.
You've got a fallacy in there as well. You're presuming that everyone who does put forth one must put forth a different and unique theory. If there are any duplications among them then your reasoning, such as it is, falls to pieces.
Since we know there is at least one scientist that has more than one theory,
On this topic?
QED "There are more theories than there are scientists."
Feel free to support your assertion by naming, say, 500 or so of the theories on the cause of the Ordovician extinction that various scientists must, by your reasoning, have put forth.