agreed, 1/3rd rule for domino's and tenons in general.
i do think you will be surprised at how fast the domino's go out, i bought a box of 100, 8x50 dominos 2 weeks ago along with my domino tool, thinking those should last a while.
but now two weeks, and two furniture pieces later the box is practically empty!
if you only need a small number, it is always possible to make your own, given that you own a planer and a router table.