The area is 18 high and 100 linear ft long The block I'm using is 6high x 8 wide x 12 deep How many block will you need?

Your wall is 1800 sq ft. Each block is 48 sq ins on the face, the depth does not affect the result. 1800 sq ft = 1800 x 144 sq ins, so number of blocks = (1800 x 144)/48 = 5400

Sorry, I misread the figures first time. Correct result is 5400.