Will this work to get out due if it has already transferred?

I DON'T SEE WHY IT WOULDN'T. I KNOW IT WORKS GREAT. I PUT 2 OR 3 IN AT A TIME AND IT ABSORBS EVERYTHING, NO TRANSFER AT ALL.

How many color catchers should I use per load?

Sophia, I use just one color catcher sheet. I usually wash small loads of clothes so one sheet has worked well for me. If your washloads are larger, maybe 2 sheets would be better but otherwise one should be fine.

Which country is this made in?

Sadly this product is made in China! Like so many things these days in this country, USA, we have American companies importing almost everything from China.

How does the size of the sheets compare to other brands? I don't understand why people are saying they use two or three sheets at a time.

Not sure why people use multiple sheets; I actually cut mine in half and use half a sheet per load. That way they last longer. For me, they work just fine at the smaller size.