You need to know the max amperage the pump draws under full load, and the length of the wire.
You want to make sure you find a chart for amperage that is automotive based, and not AC service. I would use "plenty." And go by the FOOTAGE. A 5 ft wire and a 15-20 ft run are different cats. Just like in class weights, go by the "top" or "bottom" of the "class." If you have a 20 ft run and the chart says 14, and the amperage is in the middle / top end of that box, go up one to no 12
Make sure the chart you find is for "one way" and not the return path IE "in a boat."
Wire Size Calculator
Automotive Electrical Wire - Wiring Products
Automotive Wire Gauge Guide
https://www.viaircorp.com/v/wp-content/uploads/2017/01/VIAIR_wiregauge_chart.pdf
