A:
You can create backups of any number of devices on an external drive, provided that the combined amount of files you are backing up is within the capacity of your external hard drive. (If that is worded poorly, I simply mean that you can back up as much as you like. However, you just need to make sure you are backing up less than 1 TB worth of data-- you probably are safe on that, though!)
However, "backing up" can mean either a) simply copying over files you would otherwise be unable to or would be irritated to replace (work/tax documents, pictures, music, videos, etc) or b) using an application to create a special image file that can be used to clone everything on your computer at the time at a later time.
The second option obviously takes more space but is probably the simpler option (which application you use is up to you though, naturally) but if you happen to currently have malware/viruses/etc on your computer, making a backup at that point will also carry over those same cooties when you restore your computer later.