How many slave ships operated during the Transatlantic Slave Trade?

Not that many. There was a cartel operating out of Northern France, mostly run by expat. Irish captains.

Most of them!

There was a "triangle of trade". Most ships made the same voyage and one point of that triangle was Africa for slaves.

Later some countries banned the practice and when that happened some ships became specialist in the trade.

Too many to name, but the site below has a huge list.

http://www.slavevoyages.org/voyage/search

It also varied by year, many ships were lost at sea or went down on the coastal areas.

Indeed there are no accurate records of the times between about 1520 and the early/mid 1800s. I assume that you are aware that most slaves went to the Caribbean, or to Central and South America - the trade to North America was minor.

Nobody knows. And the legal definition of what constitutes a slave ship was never agreed – a number of trial collapsed because of this.