I got a MTX Thunder amp for either one or two 12" subs, Im not decided yet how many Ill use.

1. It has 3 30amp fuses sticking in it, does dat mean it can provide about 540W rsm?

2. If it already has the 3 fuses in it do I still need an inline fuse on the wire going from my battery to the amp? If so, how powerfull should it be?

thanks

1 - Where did you get that number? It should be more like 1000rms, but there's no way to be sure

2 - The fuse by the battery is not there to protect the amp, it's there to protect the wire/car in case of a short between the battery and the amp, in which case the amp's internal fuses will do absolutely jack for you. So yes, you still need a fuse by the battery, the size of which depends on what size wire you'll be using.