A gun in Borderlands are made overall using a randomzing algorithm including some other things, that's the reason there are so many guns (the developers can't possibly make that amount of guns obviously).
Basically put, a set number is not something you can find through the internet, especially when you consider there is a case of the SAME gun having different stats on it.
The numbers definitely didn't increase drastically from each game to another, they did increase but not majorly so.
So overall, there isn't a document on the internet that will give you the correct number, however it's definitely more than the 17.7mil that borderlands 1 had. Borderlands 2 most likely increased the count with only 2-3 million due the new elements. If you are a very hardcore fan that collected a lot of guns from Borderlands 1 you will realize many basic ideas of every model carried over (if not most)... The Pre Sequel does mostly the same (especially with Jakob guns)