With the +1 to cast and the game-controlling nature of Dark Magic, a 2lv2 Sorceress army can realiably cast 2 spells of the 10-12 power level variety per turn.
I'd rather say that under the listed conditions "2lv2 Sorceress army can realiably TRY TO cast 2 spells of the 10-12 power level variety per turn." In fact, for each spell chance to cast it successfully is only about 50% (roughly).