"Do you happen to know how many Confederate regiments were populated by men from the north?"
In contrasting with Federal units from Confederate states, perhaps a better question would be 'how many Confederate regiments were raised in free states?'
Answer: None that I've ever heard of.
As for northerners populating confederate regiments, not counting those units raised in border slave states or northerners who had relocated to the south before the war, I know of at least one company from southern Illinois organized and went south and served with a Kentucky or Tennessee regiment and I suspect that there were some from southern Indiana and Ohio who did the same individually or in small groups.