Time Machine External Drive Size for 8TB

SSDs don't come in sizes over 4 TB. So try one of these from I recommend drives from OWC (MacSales.com) as they have proven reliability, excellent customer support and warranties.  Consider one of these:  OWC Mercury Elite Pro -HDD up to 20 TB. The 18 TB version looks like a good price point.

OWC is considered by most here to be the premier 3rd party hardware supplier for Macs.

Generally, 2x-3x the size of your internal drive.

But with the size drive you have, that may be a tad optimistic. In reality, you need enough space for the active data plus room to grow. But if you anticipate filing the drive, always leave 15%-20% totally unused, you're going to need a really big drive.

Personally, I would never buy a drive anywhere near that size. Lose the drive and your lose a massive amount of data and work. But that's just me; I get it about backups, but still recovery of that amount of data potentially is unsettling to me. I prefer no larger than 1TB and several external drives for data storage and backups. Just far more practical for my needs.

Generally an HDD is sufficient, but for your drive size if it's even half full is going to take a very long time for initial backup on an HDD and proportionally longer for changes as the system is used and data is created or altered. Either HDD or SSD will work, you have to decide on the practicality of either.

What you are mentioning is If your new Mac inherits your backup history - Apple Support (CA)

It is suggested New Machine with Brand New and Never been used SSD Drives with a Minimum 4 TB Capacity

I would never inherit an old Backup Drive on a New Computer.

Personal choice but I I ever needed to access Old Files on the Older Drive - out of luck could no longer do that

Put them aside for Safe Keeping or to re-purposed at a later time

To purchase a Single External SSD Drive 2x to 3x bigger than 8TB Internal Drive maybe a challenge and costly.

Though, you an purchase several External SSD Drives of a smaller size like 4 TB ( at a cost )

There is no limits on the number of drives you can use with Time Machine Backup.

Building on my previous report

If you have 1 drive setup to Time Machine Backup and add a 2nd drive to Time Machine Backup.

Time Machine Backup will Automatically backup to 1 drive and then automatically switch to the second drive on the next time it fires up the backup.

There after, it will alternated back and forth between each drive

Double the Drives and Double the Protection all without an intervention of you the Users

That sounds like a good deal to me

FYI, the suggestion form @Old Toad looks like a very option

OWC is often the Go To for proven and reliable Third Party aftermarket products