This is basically an arithmetic exercise. A "typical" brick is 4 inches wide x 8 inches long x 3 inches deep. Actual size varies with the specific brick. For the hypothetical "typical" brick each brick covers 4x8 = 32 square inches. To convert from square inches to square feet, divide by 144, so each brick covers 0.22 square feet. You need 256 square feet, so that means you need 256/0.22 = 1152 bricks.
Actually, you do not need quite this many, because you are not going to butt each brick hard up against the next one, but this is a conservative number. Of course you need to redo the arithmetic based on the actual full size dimensions of your specific brick. You need more brick if the patio is curved because you are not going to get full coverage when you start chipping bricks to make the corner, but this is a pretty good estimating start.
How much gravel do you need? Well, when we did our brick walkway, we overexcavated one foot, and backfilled with gravel. If you do the same, you will need 256 cubic feet of gravel. On top of that, you will need at least four inches of sand (1/3 foot), so that would be about 85 cubic feet of sand, which is what you place your brick in. At least, that is what we did, and it worked out very nicely. The walk has been in place for 20 years with no problem.
I suggest you purchase a book on building walkways, they are available at any big box store. There are all sorts of tips in there, including layout, materials to use, how to compact the sand and gravel, drainage design, selecting the proper type of brick, weed control etc. Well worth the investment of a few dollars.