My head unit only has 1 rca jack in the back do i need 2 splitters or just one ? to run 4 subs on my 4 channel amp. Lmk

you don't run subs on a 4 channel amp.

aside from that you will need 2 splitters. one that makes 1 Left into 2 left and one that makes 1 right into 2 rights.

Then what do i need a 2 channel ?

subwoofers run off 1 channel amps. generally 2 subs per amp unless you have 4ohm single voice coil subs in which can you can get 4 on 1 amp usually.

Well its 4 channels. It should be able to run 4 subs at 2 channels.

what amp and subs? exact models not just numbers please.

When you split rcas you split the inline voltage, dont

walmart sells rca y-splitters for like 10 bucks. but this dude right ^^ you drop voltage. get a niiiiice mono block amp. usually bridging 2 channel or more you stress the amp especially in 1 ohm situations.