I have a seven speed cassette and 3 speed chainring how many teeth should there be on the largest chainring. I am asking because I think I need to get a new chainring.
There's no universal correct number of teeth for the largest chainring. For MTB triples 42 teeth is common, while road triples usually have 50 to 53 teeth.
If you think that the chainring needs replacing because teeth are worn off, then counting isn't the way to check. Teeth are evenly spaced, so if one has broken off, then it should be easy to spot. You also need to look at the profile of the teeth. They shouldn't be too sharp or more worn on one side than the other. Google has plenty of pictures of new chainrings and worn ones if you're unsure.