Regular stones require 12 of each level to get through it. 2, 4, 6 of [1]'s is the progression for +1, +2, +3, then it resets to 2, 4, 6 of [2]'s for +4, +5, +6, etc. This goes to +25 I believe.
Somber stones for unique/special weapons are different. Their progression only goes to +10, so it's...