Get ready for a laborious job if you are going to do it yourself, and an expensive job if you send it out.
First step is the hammer out the dents. I was shown a method that works well - if you have a drill press, chock up a flat center punch and use it to precisely locate the punch on the opposite side of the dent and "push" it back so the metal is flush to slightly beyond flush, so that you can sand it down to smooth on the side that you see when the cap is on the car.
Now for the time consuming part - working up from a large grit (180 - 220 grit) to a polish 1500 grit, until you are a cut size (each size of the grit produces a correspondingly sized "cut" in the metal) that can be polsihed out with one of an assortment of polishing attachments mounted in your favorite die grinder.
It is probably worthwhile to Google the topic to find some more detail.
The trick I am told is being able to use various tools and methods to handle the various contours. On the convex shapes of the hubcap, it is a lot easier than the concave, since you cannot use a sanding block.
Advice: 1) get some stainless steel scrap parts and practice before trying it out on your hubcaps.
2) work outside if you can because it's dirty work when you start using a polishing wheel.
I've only just begun to learn how to do this work, and it is work! It's more of an artform, and you really have to be into the gratification of doing something yourself or you will put the job down and never finish it.
There's a guy in my town who does SS restoration as a quasi-retirement hobby, who has been giving me some lessons. There are also a lot of shops who will do the work for you, but get ready to climb off your wallet!