Hi!
I am making a queen size quilt, 90 by 108 and I am using 9 1/2 squares of fabric. This will sound really stupid, but I can't figure out how many I need. I am guessing 120, but I don't want to do too little or too many.
They will finish out at 9 inches.

I think you're right. You will have a 10 x 12 block layout...

:)

90" divided by 9" (finished size) = 10
108" divided by 9" = 12
10 x 12 = 120

Perfect!

yes 120 is right

Are you using and sashing between the squares because that will change things.