Summon 10 Zombies at players position, to all players

There is a way to summon 10 zombies near every player on the map with command blocks twice a day?
In the official command wiki, we can read this:
To summon 10 creepers at the nearest player's position (only works as long as there are at least 10 entities in the loaded world):
execute @e[c=10] ~ ~ ~ execute @p ~ ~ ~ summon Creeper
This command work with command blocks? I read somewhere else the execute command doesn't work in command blocks.
Also, I didn't understood why the world must have 10 entities loaded. Can you explain this to me?
Sorry if this question was alerady answered. I searched, but I didn't find anything.
Thank you for your help.
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How do you summon 10 mobs at once?
The command for summoning is pretty simple. "/summon mob". Players just need to fill in the mob name, though there are a few exceptions. The command remains the same for both Minecraft Java and Bedrock Edition, and is one of the simplest to use.How do you summon multiple zombies in one command?
the command /testfor would help you, it use to detect player and activate commands. Also you can use /summon zombie @p ~ ~ ~ To summon a zombie (or multiple) directly on the player closest to command block.Minecraft Commands: /summon Tutorial
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