"If a div is published at $10,000 it is fair to say that if you were theoretically an extra winner then you will get approx $10,000 for a unit."

If $10,000 was available for distribution and you had an extra $1.00 unit on the winning combination you would recive $5000.
You already know that.

"How do I find out how many winners there were when a particular dividend is published??"

This is not made public. You can work out how many winning units were on, but that could be one punter or it might be 50.

"I have seen trifecta divs published at $10,000 when the pool size on an internet site says the trifecta pool was $7,500 only!!
How can this be and how do I get the right figures?"

Where that occurs contact the TAB and ask them to correct the figures.

Mr. Logic