Oh yikes, this one hasn't been answered in 4 years..
Let's get the area of one tatami, then the one of the floor.
Tatami area: 6*3 = 18 sq. feet
Floor area: 48*24 = 1152 sq. feet
To know how many tatami are needed, we need to divide the floor area by the tatami area:
1152/18 = 64
You'd need 64 tatami to cover the floor