When a cleft graft puts out growth with more than one bud, do I need to remove all buds but one or leave all of them on.

Leave them.

The less buds you leave the stronger the growth will be. It also depends on whether you want to grow a tree or you prefer a bushy form. If you leave the strongest one among them, you can always get additional couple of inches of trunk, or sometimes even a lot more.

Igor

Igor, you are right but it is safer to leave several sprouts and choose the strategy later.
Because the tail of your dog can break the only sprout!

Thanks I will leave them for now and after they harden I will remove all but one

I keep the grafts (until established) in a purpose made shade box where no dogs are allowed... Your approach is more on the safe side.