I've heard of a rule of thumb to use one via per amp of current, but I have no idea what that's based on.
A more sensible rule would be to use the same copper cross section for the via, as for the tracks. KiCad has a built in calculator for copper track resistance and temperature rise. In general, copper thickness for tracks is 35um for the tracks, and around 17um for via plating. If your via's are clean (not plugged and with cutouts in the solder mask) you can also fill them with solder, which increased both their current handling and robustness.