The usual result is the used disk space. You'll get one image file per drive.
In my town and Amazon.com the USB 3.0 HDD of the 2TB size by Toshiba (the CANVIO) is under 99 bucks so figure one of those might hold 2 or more full backups.
How large of an external drive do I need to copy my entire system of a 256GB SSD and 1T HDD tocopy my entire system plus photos and documents? I previously had a 1Tb HDD and never used half of the data in 4 years of use.