Reaching the 20x score multiplier through missions means completing 20 sets of the three missions. In total you will be completing 60 different missions ranging from picking up coins, reaching certain scores, utilizing power ups, reviving, ect. Be warned that anything related to daily or weekly missions will need to be bought as the features of the game are permanently glitched.
After you have finished your 20th set of missions there will be additional missions available but they will not increase your score multiplier. Once you start your next run following the 20th set the achievement will unlock.