How many doses of MMR vaccine is to be given to 18 year old adult? And what are its side effects in them?

Depends. The routine in the US is one dose at age 1 yr and again at 4-5.It is never repeated unless a prenatal workup shows a low protection level. If you never got it in childhood, it would be reasonable to do 2 doses at least a month apart to be sure adequate response was obtained. Some teens or adults note some transient joint pain during the weeks following the vaccine.