Is it normal for your muscles to twitch now and again and can somebody tell me how rare hypomagmesia is in a healthy person?

Yes. Rare if healthy: Occasional muscle twitches are normal. Should see someone if they are progressive or frequent. Hypomagnesemia is not common in outpatients (kidney has a remarkable ability to conserve mg), but is fairly common in ICU patients. Usually has a pretty obvious cause and is not commonly tested for since clinical consequences in healthy outpatients are generally not severe.