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The supply of a unit is the maximum amount either player may buy during a game. Each player has their own supply.
Units may have supply limits of 1, 4, 10 or 20 (which are sometimes referred to as 'Legendary', 'Epic', 'Rare', and 'Common').
Generally, the lower the supply of a unit, the stronger its potential, though there are exceptions. For example, Amporilla is 4-supply but much more likely to determine the outcome of a game than Mega Drone, which is 1-supply.