How much power you need, depends upon which subwoofer you choose. You'll find a recommended power range (in continuous or RMS watts) listed in the specifications. The lower number is, the absolute bare minimum amount of wattage you need to get acceptable results. Typically,  more power is better because bass consumes a good deal of power.  To really get results from your subwoofer, choose an amp rated in the upper third of your subwoofer's maximum RMS power range. This is something to consider when you’re looking at the all-in-one under-seat subwoofer since they include the amp. Your amp's RMS output can even exceed the woofer's maximum RMS rating, but you'll need to keep an ear on the sound quality because if the bass starts to distort, you'll know that you've reached the top end of your amplifier to the subwoofer performance capability. Turn the volume down a bit until you notice the distortion clears and you have a clearer sound. If you crank it too high, you can damage your subwoofer and speaker.