Since the battery is in the trunk you'll be fine with 10. If you wanted you could go for 8. Since the battery isnt up front theres no need for 4.
It would also be easiest to run the RCAs under the drivers dash then under the kick panel and door sills.
Do you have a the factory HU or a CD player? The factory is removed with a special screw head that would be easiest to go to the dealer and ask to borrow. Otherwise you'll need an extensive tool set and hope you have the right head. If its aftermarket, just take off the faceplate and undo the retaining clips.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>