If plywood sheets sell at 4ft. x 8ft. a sheet and I want to and I want to build a 12ft x12ft kitchen ; how many do I need to buy? Can anybody tell me how to set this problem up?

I assume you simply want to use the plywood as a subfloor?

You would lay 3 sheets side by side to cover an area of 12 x 8.
then turn one more sheet sideways, leaving a 4 x 4 area open. You have to cut one more piece in half to cover that area.
So you would need 5 sheets.

In reality, you would not lay them they way I suggested, but they would be laid in a staggered fashion to give more stability. You would do it in a "brick-layered" pattern.
Start with a whole sheet, then put a 4 x 4 sheet to finish.
Then start with a 4 x 4, and finish with a whole sheet, etc.

YOu still need 5 sheets