Yes, the 5-leaf clover is a mutation like the 4-leaf clover that does appear occasionally, but less common than the 4-leaf clover. There are 6, 7 and other multiple leaf clovers that sometimes occur.
What are the superstitions?
The superstition for the 5-leaf clover is extra good luck and attracting money. The 6-leaf clover is sometimes believed to symbolize fame. I have not come across any published information on the superstitions of other multiple leaf clovers.
What is the frequency of the 5 and other multiple leaf clovers?
Since the 5-leaf and other multiple leaf clovers are random mutations and affected by weather and growing conditions, any individual studies would probably be meaningless