Plums are very nutritious and contain the needed vitamins that our bodies need to stay healthy.
So there's nothing wrong with eating as many plums as you want too as you'll just improve your healthy because of the vitamins and minerals contained in the plums.
However if you do eat too many plums you could get a stomach ache or have to poop more often.
You might also get some diarrhea if you do eat too many plums but other than that there's nothing wrong with eating a lot of plums.
If you're having diarrhea or stomach aches or need to poop too much then you're probably eating too many plums and should slow down on the amount of plums you eat.
I eat around a cup of prunes every few days as a snack and they're very good and healthy and I never had a problem.