Milan,

Zero plays two roles in our number system. First it is used in our place value notation to distinguish between five-hundred and six and fifty-six (506 and 56). In 506 it tell us that there are 5 hundreds and 6 units but no tens. But this is not the use of zero that you are questioning, it is zero as a count of "no things". A counting number is an abstract notion we use to specify quantity as you indicated in your question. The number five is the quantity of fingers I have on my left hand or the quantity of Es in the "word" FEREBEEE.

It is extremely useful to include zero as one of the counting numbers. For example it allows me to answer the question "How many Gs appear in the word FEREBEEE?" My answer to your question "Why do people call 0 a number?" is that it is extremely useful to do so.

Harley