When you summon a creature, the summoned creature’s level cannot exceed the level of the creature summoning it, unless the summoning effect explicitly says otherwise.
Each player has a Summon Cap: the maximum number of summoned creatures they can control at the same time. The default summoning cap is 1, increasing to 2 at 9th spellcaster level and 3 and 18th spellcaster level.
If you would gain control of summons beyond your Summon Cap, you must immediately choose which summons you keep.
Any summons not kept immediately end (they disappear), unless a rule specifically says they persist.
A summoned creature follows all of the rules below unless an effect says otherwise:
Actions
You expend an action to command a summon. Allowing it to do 2 actions. Your summoned creature can use reactions, provided you use your reaction.
Duration / Maintenance
A summoned creature disappears if you are no longer maintaining the effect that summoned it (for example, if you stop Concentrating or you stop Sustaining, depending on the effect).