Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
What do you need to plug into the USB hub? If there are several devices, or any of them draw a lot of power (e.g. external HDD), you should get a powered hub. If you just need a few data cables for connecting to other devices, an unpowered hub is probably sufficient.
 
I have a 2017 iMac 21.5. I which to add a hub. Should I get a unpowered one or one with it's own power adapter?
Nobody can answer that question unless you tell what you wish to attach.

If all you want is USB, Anker has some nice USB hub with 3 charging ports and 7 normal ports. Obviously it's powered.

Be careful and read any offers very carefully, because some sellers think that a "USB-powered hub" (a hub that takes its power from the USB port of your Mac) is a "powered hub" (one that connects to a power outlet).
 
I intend to plug in a second external drive for SuperDuper or Carbon Copy Cloner. What else I don't know. I just don't know for the moment. I just don't want all my ports taken up.
 
In general, drives should always be plugged into powered hubs. Depending on the drive and the hub, you might be able to get away with an unpowered hub, but I would recommend a powered one.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.