How many f-5 tornadoes were there in April and May in 2011?

There have been estimated at least 109 F5 tornadoes in the U.S. Since the introduction of the Enhanced Fujita scale there have been 9 EF5 tornadoes, which is essentially equivalent to an F5.

Note that this number should be taken with a grain of salt as it can be difficult to distinguish between F4 and F5 damage, especially in records that must be evaluated based on historical accounts.