Players are able to summon multiple monsters at the same time as long as they're from different GROUPS (1, 2, 3, 144, 145, 146). It would be useful if players used as many monster summons as possible during the following events: Meteorite, Mobilization for War, Treasure Island, and Anniversary.
This list separates monsters by group (1, 2, 3, 144, 145, 146), and how much time is on their timers once summoned. It also includes where their orbs can be found (monster quests located at the bottom of each quest land and gifts), and how much energy is needed to summon (including energy needed for monster quests).
Group 1 is monsters that can only be summoned with orbs from monster quests.
Group 2 is dragons and serpents.
Group 3 is misc monsters, but most can be summoned with gifts.
Group 144 is Thanatos of Fire & Ice.
Group 145 introduces monsters with resistances (most are elemental but some are regular physical resistance) and stance monsters.
Group 146 is nightmare monsters, which are stance monsters that can be made easier with pets that meet pet stance requirements.
*You might not be able to summon monsters from groups 2 and 145 while either is active due to a glitch. Some players have been able to, others haven't.