If it's pretty long leg sinew, 8 tendons should do it. So a mature deer will provide you with half of what you need. Smaller deer might take more and front legs are generally shorter. It also depends on how thick you want it. I'd go at least 2 and better yet 3 courses (or layers).
Back sinew is easier to process but will take a couple more deer's worth since you generally only get 1 thin piece from each backstrap (2 per deer). You'll want at least 8 strips or 4 deer's worth.