By a bunch, do you mean like 4 or 5? Reece cups are milk chocolate so not as dangerous as baking chocolate or dark chocolate. It's possible he'll be fine, even though he's not a large dog, and it depends how much he ate.
You could make him vomit if he ate them within the last 3 hours. Use hydrogen peroxide, ONLY use 3% strength, NOT hair colouring strength.
My dog ate Cadbury Buttons, nearly a whole pack a few years ago. I didn't realize it at the time, it was only the next morning I put 2&2 together, as a whole lot of chocolate from a dish had gone missing and she was sick. She had an upset tummy and slightly bloody diarrhea. I fasted her (after getting advice from the vet) all day with just water, and she was absolutely fine.
My dog got into a shopping bag, which had easter candy. He ate about a baseball sized amount of chocolate, and didn't show any signs of distress. I also checked his stools, and he had no problems there as well.
It was like it never even happened. He lived, and is still happy as ever.
I know how you feel, when I first realized he ate it, I got scared and told my husband we should call the vet.
If you feel like you should call the vet, do not hesitate. As the owner, you will know if something is wrong. I've called the vet, because my dog got into chicken bones, and ate some. My vet is nice enough to awnser the questions I have over the phone.
Good Luck to you and your Dog!
It's true that milk chocolate isn't nearly as dangerous as dark chocolate, but if your dog is very small and ate a whole bag of Reese's cups I would call an ER vet for advice. The ER vet near us is very good about giving advice over the phone rather than insisting that you come in before discussing the problem. If they think it's a true emergency, they'll tell you.
Something to be somewhat more concerned about than the chocolate itself is the amount of fat in both the chocolate and the peanut butter. Some dogs get ill from the sudden ingestion of a great deal of fat, which can cause pancreatitis.
Years ago, our two 55 pound mixed breed dogs grabbed two bags of Hershey's kisses off the far reaches of my kitchen counter. I wish I had a video of the incident because I can't imagine how they counter-surfed that far. I got home from work and found the empty bags on the floor. Everything was gone including the foil wrappers. Those began to appear the next day. There were sparkly little Christmas poops all over the yard for a few days. Both dogs showed no ill effects whatsoever from their overindulgence, and in fact were mightily pleased with themselves.
Chica, our shepherd mix at the time, had to be the one to snatch the bags off the counter. She was quite the agility dog and I should have known better than to leave anything at all even slightly within her reach. One grocery store chain used to carry the BEST French bread. It had to have a pound of butter in each loaf. (I'm still mad they went out of business!) I came home from work one evening ready to start making dinner and I find the bread missing. Thought I was losing my mind or maybe the bread didn't make it into the grocery bag when I bought it the day before. Hmmm... no evidence in the house, so I figured it was my error. I never suspected the dogs who ate their own dinner with their usual gusto.
A few days later, the same thing happened again, only this time the dogs were busted. There was one piece of the paper bread wrapper about the size of my palm left on the living room floor. The little beggers had themselves a regular feast - twice no less! - and not only ate the entire loaf of bread but the bag as well. No ill effects from those excesses either. Crazy animules! :-D