According to PnP animate dead allows you to create a skelleton or zombie from the corpse of a small or medium humanoid. It lasts 24h and has a casting time of 1 min (means only outside of combat). Its a lv3 spell and casting it at higher levels gives you control over more corpses. You can give all of them the same command with a bonus action. It does not require concentration.

There are several conjuration spells (some of them allow several summons at once) that require conjuration.
Losing concentration means the creature becomes hostile.

So a high lv mage could have many summons by using animate dead and conjure minor elementals, since both spells allow more creatures when you cast it at higher levels.
The elementals act themselves, the mage has to use a bunus action to give all undead the same command and they do this until the mage commands something else.

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