For sub woofers do you need a mono amp or a 2 channel amp?

Depends if you are building heavy bass system with quite a few subs then a mono amp would be suggested but if it is just a single sub just for a standard bass upgrade then a 2 channel amp would be fine you can still use a mono amp on one sub

just remember to take into perspective the RMS rating and the Impedance on both the Subs and the Amp