As humans, it seems we could have had any number of fingers and toes but somehow we ended up with 5 fingers on each hand (also known as pentadactyly) 10 fingers in total. What's so special about 10 ?
Around 367 million years ago during the Devonian period the first animals began to walk on land. These animals belonged to the superclass tetrapoda (from Greek: τετρα- "four" and πούς "foot") which contain the four-limbed vertebrates known as tetrapods.
They were primarily aquatic animals living in shallow waters. Fossil records indicate that they had up to 6-8 digits until 360 million years ago when they began to transition from an aquatic environment to a terrestrial one.
During this environmental transition, the 8 digits of the tetrapod were reduced to 5. No one really knows why but It's probably a good trade off between the hand being flexible enough to manipulate many different elements of the environment and being overly bulky or expensive to create and maintain.
In evolution, If a trait neutral or provides no evolutionary benefit it tends to get selected against. They eventually lost digits during the transition, five digits per limb was the optimal for the circumstances of our ancestors and so it fixed in our DNA.
If we had evolved from ancient fish with only 3 bone formations where our ancient ancestors had 5, we would only have 3 digits, rather than 5 digits per limb.
Losing things in evolution is generally relatively simple, yet re-gaining them is hard. So while there is still a tendency for evolution to create species with less digits, there are no examples of land dwelling species with more than 5 digits.
There are however rare genetic cases of polydactylism, a rare anatomical variation where humans have an extra finger or toe. It affects 1 in 500 humans and occurs because of a random genetic mutation or one that is passed down from parent to child. These extra fingers do not work.
References:
Why do most species have five digits on their hands and feet? - article from Scientific American website
Ask evolution: Why do we have five fingers? - article from SBS website
Polydactyly - article from Wikipedia