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A single layer of bricks is intended to be laid as a veneer wall (meaning against another wall, like on a house) In that case the opening has to be closed with e.g. 4'x8' o.s.b. or plywood on 2X4" studs and then a wrap and then the brick wall is laid.

Or do you want to close the opening with a single layer of concrete blocks 4" or 6" thick. What are the existing walls of the garage made of?

Are you building it or having it built? If you're building it you can determine the square footage of one brick and divide that into the total square footage to see how many bricks you need. If someone else is doing it, you may get quite a few different answers.