Hey Radarithm, sorry to chime in late but wanted to give my two cents because I've studied from those exact books, and maybe can help you out. When I was doing Kleppner I struggled a lot but managed to do a good number of the problems (I'd say around 90%). The ones I couldn't do I left for later, and every once in a while I dig back through the book to see if anything's changed (usually I manage to solve at least a few more). As for Purcell, that's a different story. Try to do as many of the problems as you can (the ones with answers in the back, so you can check your work), but don't worry if you can't do some *** problems (I usually can't do them, tbh). Try to be able to do all (or most) of the **, and obviously all the *. Also do some exercises (same as problems but no answers in the back), but if you can do most of the problems you're probably fine (I think there's about 30 problems per chapter, so that should be enough without doing any exercises. It's up to you though). Like Wannabe said, though, there isn't really a magic number or anything for what you should do. Have fun learning from K&K and Purcell, it's really fun.
EDIT: Forgot. As for if it's important to get the correct answer- yes and no. Yes in that obviously, if you get the right answer you most likely understand the concept and can apply it. But also no, because when you get the answer wrong you realize you didn't understand something fully, and can go back and review it. NEVER just look at the correct answer and process straightaway if you get it wrong. Try again, or peak a little at the solution to see the step you may be missing (I think Purcell may have talked about this in the preface, it's really important).