If you really want to know the exact number of rotifers you have, I would recommend purchasing a hemocytometer. These were designed for the medical field, in order to count blood cells. Here is a link that gives directions on how to use a hemocytometer for counting cells ... you would use the same method for counting rotifers.
Using a Counting Chamber. And here is an online hemocytometer calculator that is helpful.
hemocytometer calculator.
Luis' statement, that you need at least 2 rotifers per ml, means just that ...
at least 2 rotifers per ml. Having one hundred rotifers in a single ml is very possible. The more you have, the more attention you need to pay to feeding them and making sure they do not crash the fry tank.
(I am sure you know this, but just to clarify, 1 ml of water is equivalent to a single drop of water.)