"decimate - remove one in ten (decem)
undecimate - remove one in eleven (undecem)"
The Romans themselves were pretty imprecise, though. For instance a centurion was notionally in charge of 100 soldiers (a century...) but in fact centuries were almost always less than this, with the top centuries of a legion perhaps being 90, and lower ones 80 down to 60. After a battle, of course, the numbers were smaller still.