You can do any wiring you want using an amp, or not. So if you only want an amp on the sub, for example, and you want the rest of the speakers to be connected to the speaker outputs of the head unit (without using an amp for them) that's fine. You have options, which is the nice part of all of this.

Different head units have different quantities of RCA outputs. I think that's where you're getting hung up. Low end head units often have a L and R only, but better models will have 4 or 5 or 6 or 7 (often 4 for front/rear plus an extra for the sub, making 5 total). With seperate RCA outputs comes seperate control for fade, balance, frequency cutoff, etc., which is why there are more than just a single pair of left/right RCAs.

Take a look at the specs of the head units that you're interested in; that will tell you a lot about what's available on the market right now.