AS a Junior Manager on salary! How many hours do need legaly between shifts?? Is it 8 or 12 hrs

Eight hours on, l2 hours off.

Assuming that by salaried, you mean salaried exempt, there is no legal restriction in California where I live and I haven't heard of any state that has a restriction.

"Exempt" pretty much means they own you, so you should try to get a salary that reflects that.