There are actually fossil records of creatures with all manner of digits - more than 10 per limb IIRC and down as low as two. However, five seems to have been a remarkably successful number for fingers on the whole - and nature tends to go along with the rule "if it ain't broke, don't fix it".

Not quite. It is true that early vertebrates had a great variety in the number of digits on each limb. But the fact that we all ended up with five may have been an accident, not a result of natural selection. The other lineages died out, and the one that was left just happened to have five digits on each limb.

I suspect the specific number of digits isn't a big deal for most critters. Horses lost four of theirs, and get by fine with just a single toe on each foot. Humans sometimes are born with six or seven fingers/toes, and it's usually not a problem (unless it's a result of some deeper developmental defect). Cats with six or seven toes are very popular with pet owners, and so there's been a sort of selective breeding for polydactylism in house cats. And in Stephen J. Gould's famous example, the panda has five fingers and a thumb. The thumb is actually a modified wrist bone, because the five fingers were too evolved toward running to become opposable.

Because of this variation, I suspect that five digits per limb is an ancient trait, inherited from a primitive common ancestor, rather than a body plan that was particularly advantageous.