How many frames do they use for stop motion?

It depends, some not that detailed videos take about 10 to 100 frames, but then again, some of the more detailed videos can have up to 1,000,000,000 frames!!

Think about it, have you ever seen "Fantastic Mr. Fox"? well that movie is ALL stop motion animation, and the movie put together is about 2 hrs and 30 mins when you estimate it. And it took over EIGHT YEARS to take all the pictures, hope this helped,

Sincerely, Ben Kohan