It's common for polys to only have extras on the front. In rare cases, the back feet are poly too.
Our poly has odd numbers, 6 on one foot and 7 on another, but he still has 4 on the back. They can have up to 8. In the case of 8, they effectively have 2 feet on each leg. On our cat, the foot with 7 actually has a half-sized pawpad where the 3 extra toes come off. It's like he has 1.5 feet on each leg.
Hemingway was famous for keeping these cats, hence the name.
You'll also find when you do your search that the gene comes in two forms, the "mitten" form where the toes are on the thumb side and a seperate from the main paw, and the "hamburger" form where the toes are part of the main paw and extend from the pinky side. The hamburger gene is actually unstable and causes birth defects in later generations. The mitten gene is a little more stable, though there is some research that the gene is actually "add a toe", so breeding 5 toes to 5 toes gets you 6 toed cats.