The part number for the alternator you need is 12317560985. It's the 180 amp alternator. I agree...it can be very confusing, and I have been thru all of this recently. Instead of just buying another alternator, you can also just replace the voltage regulator for less than $40 and that may fix your issue...it did for me. Here is the link to the Bosch voltage regulator you will need:
https://www.fcpeuro.com/products/bmw...ch-12317561939