I was messing around, and in LC my Torchic used sub and Onix used rock blast. First hit broke sub, but it didn't hit 1 or more times! What happened!?
I'm not exactly sure what your asking due to the way the question is written,but any multi-hit moves work like this against a substitution = it will strike whatever number of times it is going to regardless of the substitute,additionally weakness is still counted and considering Onix gets stab on rock blast and Torchic is fire type the first hit should have broke the sub and then any subsequent hits would still fire.
Substitude withstands the hits, but when a multi-strike move breaks the Sub, it won't continue to attack. So that means when Onix's Rock Blast (Any turn, but in your case the first turn) finished Substitude, the Rock Blast ends.