Us it true if a man has a certain number of kids he don't have to pay child support anymore..

Depends on where in the world you live. I know of a man who has 13 children and has to pay support on all of them.

We live in Florida... I was told 7 by someone and I have also heard 10...

It may be true that AFTER a certain number of children you don't have to pay ADDITIONAL support.

But having children you can't afford is irresponsible. So I'm not even going to look this up. Instead I'm going to ask why you are asking?

I was out the other night and over heard people talking about it... so I was curious...

The answer is "no".

"Florida Statutes
61.30 Child support guidelines; retroactive child support.—
...
(12) ... (b) Except as provided in paragraph (a), the existence of such subsequent children should not as a general rule be considered by the court as a basis for disregarding the amount provided in the guidelines schedule. The parent with a support obligation for subsequent children may raise the existence of such subsequent children as a justification for deviation from the guidelines schedule. However, if the existence of such subsequent children is raised, the income of the other parent of the subsequent children shall be considered by the court in determining whether or not there is a basis for deviation from the guideline amount.
..."