i need to upgrade my power wire for my subs and i was wondering how much wire will i need
Are you talking about the power cable from the battery to the amp? If so best thing to do is just measure if you plan to buy off a roll. Use what ever you have string, bit's or wire connected together ect, and just run it through and then take the measurement. Remember power cable may not always fit, or be as flexible around certain spots so to allow for that also.
Id just grab a wiring kit personally mate.
I picked up a 4ga one off eBay for $40, Came with tonns of cable, remote and 2 RCA's