You need to get the set of 18. I got 10 and that was not enough for some of the workouts in STS. I had to move some of them from on DB to another, as a previous poster stated, get the 18 so you can set up all your weights in advance. I will buy more before I do another round of STS.
It is so worth the investment. My DH (Who is not doing STS) loves them also because of the incremental changes he can make on his weight selection.
Brenda in Indiana