I want to put away my fears of stomach cancer so can someone give me a estimated percentage of stomach cancer occurrence in their 20s?

Zero percent. The estimated percentage of stomach cancers that occur in people in their 20's or younger, is zero percent. A research study from the U.K. Found that about 8% of cases were in the age 36-55 yr old range, and about 92% were in the over 55 year old range. Zero percent were in the 0-35 yr old range. If a person has relatives with stomach cancer before age 55, he should see his doctor for evaluation.
Almost none. It is not common at all. If you worry see a doctor and get and upper endoscopy.