Take the dimensions of your project and figure out which direction you will run your stamps.
Next measure what your longest run will be. Ideally, you want enough stamps to cover your first row and 2 stamps for the second row. This way you will always be interlocking the stamps and not just trying to line up the grout lines by eyesight.
For example, if your patio is 12×20 feet and you want to run your stamp pattern in the 12-foot direction, you will need enough stamps to line up for 12 ft and then a couple more stamps for the next row.