Time Machine Backup
Does the iMac 24 come with a built in backup? If not what memory size of backup is recommended?
iMac, macOS 10.13
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No, macOS comes with backup software called Time Machine.
see > Back up your Mac with Time Machine - Apple Support
You will need to get and use an external drive for Time Machine backups.
see > Backup disks you can use with Time Machine - Apple Support