Is it possible to summon multiple mobs at once?

This answer's source is MinecraftForums. You can find the original thread here: http://www.minecraftforum.net/forums/minecraft-discussion/redstone-discussion-and/350423-multi-summon-summon-multiple-entities-blocks-w

With normal /summon you cannot spawn multiple mobs at once. But you can do the {Age:6000} tag.

To summarise the thread:

You can add the tag Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie, to add an entity. So for 16, you can paste this 16 times (or copy this below):

/summon ~ ~ ~ Zombie {Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie,Riding:{id:Item,Age:6000,Item:{id:1},Ri­ding:{id:Zombie}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}

• The } spam is because you have to type 1 } for each Riding: tag.

But, all the items will not be able to be picked up though. You can do /kill @e[type=Item] to destroy them.

You could possibly make a command block on repeat mode and flick a switch on it! That would make whatever your summoning multiply by 100 in a matter of seconds.

You can use this command to summon 2 zombies:

/summon falling_block ~ ~1 ~ {BlockState:{Name:activator_rail},Time:1,Passengers:[{id:command_block_minecart,Command:"summon zombie ~ ~ ~"},{id:command_block_minecart,Command:"summon zombie ~ ~ ~"},{id:command_block_minecart,Command:"kill @e[type=command_block_minecart]"}]}

You can use this command to summon 3 zombies:

/summon falling_block ~ ~1 ~ {BlockState:{Name:activator_rail},Time:1,Passengers:[{id:command_block_minecart,Command:"summon zombie ~ ~ ~"},{id:command_block_minecart,Command:"summon zombie ~ ~ ~"},{id:command_block_minecart,Command:"summon zombie ~ ~ ~"},{id:command_block_minecart,Command:"kill @e[type=command_block_minecart]"}]}

and more, just by adding

{id:command_block_minecart,Command:"summon zombie ~ ~ ~"},

after the last one.

This command is good, because it can support NBT tags to your mobs, have multiple different mobs and set your mobs location. However, it isn't that easy.

UPDATED FOR 1.13+

this will spawn 10 Zombies

/summon minecraft:zombie ~ ~2 ~ {Passengers:[{id:item,Passengers:[{id:zombie,Passengers:[{id:item,Passengers:[{id:zombie,Passengers:[{id:item,Passengers:[{id:zombie,Passengers:[{id:item,Passengers:[{id:zombie,Passengers:[{id:item,Passengers:[{id:zombie,Passengers:[{id:item,Passengers:[{id:zombie,Passengers:[{id:item,Passengers:[{id:zombie,Passengers:[{id:item,Passengers:[{id:zombie,Passengers:[{id:item,Passengers:[{id:zombie}]}]}]}]}]}]}]}]}]}]}]}]}]}]}]}]}]}]}

to add more paste before all the "}]" at the end

,Passengers:[{id:item,Passengers:[{id:zombie}]}]

in the 1.13+ format, you can use '/execute as @e[limit=16] run summon zombie ~ ~ ~' -- this will work as long as there are at least 16 entities in your world at the time. What it does, is it executes as every mob, with a limit of 16, and therefore every mob up to 16 will summon a zombie for you. The position of the zombies will have nothing to do with where those helpful mobs are, so you can set the position wherever you want and all 16 will spawn there. If you need more entities for it to work, you can throw a few items or spawn a few pigs or something.

Take this command for an example. It summons an item entity with four zombie passengers.

summon minecraft:item ~ ~ ~ {Passengers:[{id:"minecraft:zombie"},{id:"minecraft:zombie"},{id:"minecraft:zombie"},{id:"minecraft:zombie"}]}