The number of speakers depends on which audio trim the car came with. The premium has 8, and the HK system has 10. The 318i/is/ti versions came with only 6 speakers unless a higher package was selected, while most 325/328s basically had the premium as standard, as I have yet to see one without the 3.5" in the door.
They are midrange, but unless you have subs then the 5.25s are also going to be your lows. I suggest a sub or two or at the least a bass tube if you are redoing your factory system, as none of the speakers in the cabin are big enough for effective bass reproduction.