When you have the entire variety. That means one in each caliber from 45 to 58. You also need a 32 for for small game. Then you have to have at least one flintlock. To avoid having more than one that is relatively the same, be sure to mix-in different twists. There's 1-20, 1-28, 1-38, 1-48, 1-60/66, 1-70.
When you get to around 10, you got close to the complete lineup.... barrel lengths excluded.