How many top contributers are there in each category?

As many as will qualify there is no set number like the top 10

The Yamster says:

A Top Contributor is an Answerer whose Q&A shows that they are knowledgeable in a specific category. You can become a top contributor in up to three categories. The better your contributions are in any respective category, the more likely you'll become a Top Contributor for that category.

The top contributor badge is dynamic —you earn it or lose it depending on your recent participation in a particular category. The more actively you contribute, the more likely you will earn a badge. Just like featured users or category leaders, it's another way to feature community members who might otherwise be overlooked.

We basically take an Answerer's overall contributions on Answers in each individual category, toss them into a hamster wheel, and the ones that manage to stay on the wheel become Top Contributers.

The award of a Top Contender tag is based on your contributions in 1 - 3 categories and the quality of your answers. Being in the Top 10 list in a category is not a factor. Quality of Answers are most important along with making regular contributions.

I don't know but alot of them aren't even listed as one of the Top 10 Answerers.

Hard to say. As long as you hit minimums, you could have a 100 in each catagory

don't know for sure but I would venture a guess that there is only one "top" in each makes sense wouldn't you say?

Quite alot I think. There's to many to count. :)