Question

Frogs produce a huge number of gametes at one time - why do you think this is?

Answer 1

Their mode of fertilization is external. 
They lay eggs in the water and there is a chance of destruction of these gametes by water current if they are in less quantity. 
So they lay more and more eggs in order to rule out the possibility of destruction of all eggs. 
Thus at least some eggs are fertilized by sperm and produce zygote.