Should I have an internal back up disk in iMac for the TimeMachine?

Recent Mac models do not have two storage drives. Even with older Macs that may have had a second drive, backing up to an external drive was a better idea so that both the Mac and the backup wouldn't be lost, stolen, subject to a malfunction, or physically destroyed at the same time which would cause the loss of both the original and the backup.

Also see: Back up your Mac with Time Machine - Apple Support

and

Backup disks you can use with Time Machine - Apple Support

I'm not sure exactly what was offered or possible with 2020 iMacs but I strongly suspect it doesn't include a 2nd internal drive. You can use disk utility to verify. See: Intro to Disk Utility on Mac - Apple Support and select your operating system in the top left of that support article. You can find what operating system you have at Apple menu > About This Mac.

However even if a 2nd drive is possible, I would strongly discourage using it for a backup. If your iMac is lost, stolen, subject to a malfunction, or physically destroyed, with a backup on an internal drive you would lose both the original and the backup.

External drives are relatively cheap.

Another option since you have available iCloud storage would be to consider the following.

Apple offers iCloud Backup for iPhones and iPads but not for Mac computers.

It is possible to sync data from a Mac to iCloud if you turn on those options (e.g., iCloud Photos, iCloud Drive including the Desktop & Documents option). Syncing has some of the benefits of a backup (and the benefit of being offsite) but is not the same as a backup. Major differences include the fact that not everything on your Mac can be synced and the fact that if you delete something, accidentally or otherwise, there is only a limited time period (30 days in most cases) during which it can be recovered.

Also see the Apple article: How to back up your Mac - Official Apple Support

And more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

Access and manage your files in iCloud Drive - Apple Support

and

Add your Desktop and Documents files to iCloud Drive - Apple Support

If you're setting up a good backup system, the idea is to back up to multiple destinations, with at least one of them usually being off-site. Say,

• Two or three external drives that you rotate, with one usually being stored in a bank safety deposit box or a safe place at a trusted relative's house,
• One or two external drives and a cloud backup service

If your originals and backups are all in one place, then if a fire or natural disaster destroys that place, or a burglar steals all of the computer equipment they find there, you get wiped out in one blow.

Having everything "back up" in real time can be a bad idea, too. People sometimes think mirrored RAID sets are backup. But a fire can take out both disks at the same time. If ransomware attacks the computer using the RAID, the RAID will replicate the damage in real time. The RAID only protects a specific failure: mechanical failure of a drive that leaves the other drive intact (and available for use while the RAID is being fixed).

A 2020 iMac has a single internal flash storage drive. A 2019 iMac has a single internal flash storage drive or Fusion Drive.