Concentration

A spell caster can concentrate on a number of spells equal to their Intelligence modifier, and their Intelligence modifier also dictates the highest level spells you can have multiple concentrated, ie; if you have an Intelligence of 16, you can concentrate on 3 spells up to 3rd level, if you cast a concentration spell of 4th level or higher, while you are concentrating on 3rd level or lower spells, it’s duration either becomes instantaneous or you lose concentration on all lower level spells.