How many 15.5 X7.5 pavers will it take for the base of a 6X9 shed?

You need at least 70.

9ft = 108 inches. 6ft = 72 inches. Remembering to round up:

108/15.5 = 7

72/7.5 = 10

108/7.5 = 15

72/15.5 = 5

So you need either 10 pavers wide and 7 high (70 total), or 15 pavers wide and 5 high (75 total) if you turn them sideways.