If u buy 2 or 3 idols, summon minions, then sell this idols and buy less (like u buy 3 then sell and buy 1) u will see what u can summon minnions what u havent and cant what u have...try and u will see this 
(if u try to summon what u havent, dg will cast but nothing will hapen, but u simply cant summon what u have)
