The 30A fuse on there is a obvious give-away that 30A is the max draw that amp will see. 8 Gauge is good for 50A at 20ft, you'll be fine with that. IF you might upgrade later or add another amp, then go with 4 gauge to save you time later

Previous Owner of 00 SVT Contour #1077/2150 95 Contour SE '01 3L