Current drawn by brushed DC motor
The maximum current the motor will draw is the stall current. And if you want your motor to be robust, you will rate it so that it can survive being stalled, so you will use a wire that can carry the stall current.

The stall current is simple to calculate : it's V/R where V is the supply voltage, and R is the resistance of the winding (ignoring brush resistance. You can include that if you know it)

To calculate R you need to know the length of each turn * the number of turns, and the resistance per metre (or per km) of your chosen wire, which should be somewhere in the same datasheet that tells you its rated current.