How much space is needed for upgrading monterey?

Empty Space Requirements  >> If upgrading from macOS Sierra or later, your Mac needs 26GB of available storage to upgrade. If upgrading from an earlier release, your Mac needs up to 44GB of available storage. 

Would suggest at least 50 GB Empty Space to be more safe.

Apple says:

If upgrading from macOS Sierra or later, your Mac needs 26GB of available storage to upgrade. If upgrading from an earlier release, your Mac needs up to 44GB of available storage.