Depends on the gun. If it's something not serious and just for my collection, 1 magazine is fine. If it's for social work, I learned long ago that 3 is the magic number. If you have two magazines and you lose one or it busts, then you're down to one mag. If you have three and something happens to one, you have two.
It's the old "two is one, one is none" thing.
So, 3 is my minimum, 5 is my ideal number.
For revolver speedloaders, I don't really care for them. One is fine, after that I'll rely on speed strips and a two round speed reload.