Multi-spell Concentration (5e Variant Rule)
From D&D Wiki
Creatures can concentrate on multiple spells at once, up to a maximum number of spells equal to its spellcasting modifier. If a creature casts a concentration spell when concentrating on its maximum number of spells, it breaks concentration on one spell of its choice. A non-spell ability that require concentration is treated as a spell being concentrated on for the purpose of this rule.
When concentrating on multiple spells, each spell increases the save DC to maintain concentration by 2. A creature concentrating on 2 spells has a +4 save DC, +6 for 3 spells, +8 for four spells, and so on. When making saving throws to maintain concentration, each spell is rolled for separately.
A creature concentrating on multiple spells must use a bonus action or forgo any movement on each turn to maintain concentration. If it does not, it breaks concentration on all spells. If it is maintaining concentration on the maximum number of spells that it can (i.e. its spellcasting modifier), it must instead use an action on each turn to maintain concentration.