Whar are the realistic chances of stomach cancer in my 20s?

Very low, but...... Just based on age alone, the risk for stomach cancer is very low. However, other factors such as family history, tobacco use (of any type), alcohol use, consumption of spicy foods, and certain environmental/industrial exposures may increase your risk. See your doctor if you are still concerned.
Very rare. Gastric or stomach cancer is not that common in the us, and is rare in young people. Gastritis or stomach infections are more likely in younger people.
Very low. The incidence of stomach cancer has declined in the us. Unless one has a strong genetic predisposition, the likelihood of a person in 20s developing stomach cancer is very low.