My dog just ate about 20 chewable tums should I be concerned?

You are lucky this time. TUMS are just calcium carbonate and shouldnt hurt the dog. You might want to check with the Vet but if your dog is an average sized dog or larger he should be ok. Keep meds out of the reach of animals the same as small children. They are attracted to the fruity or minty smell and think it is good to eat and some things are very dangerous for them. Do check with your Vet but I think the dog should be ok.