[DonRotolo]

1. Just as you can define a single motor, you can define several motors. You do this for left and right already, right? So define many motors for a side, and when you send the value to one motor, repeat that same line of code to send it to motor 2, motor 3, and so on. That sends the same drive value to multiple motors.

Note that there may be more efficient ways, depending on the programming language, but brute force will get it done.

2. Asphurley67 stated, do a little math for P by following your code on paper, and see what multiplier will give you something reasonably in range.

When trying something new, it never hurts to step through each line of code by hand, on paper, to see what will happen. Lots of bugs are found this way...