If you've already used all 6 satellites and have still not attained maximum population, please check whether your Earth is already at Lv.7. If Earth is not yet at Lv.7, Please go to Social > Ranking > Find Friends to add more friends to Walkr. Once you've reached 80 friends, you'll be able to upgrade Earth to Lv.7 and increase your population! (For each 5 friends you add on Walkr, Earth's population will increase by 1.)
Please note that the Green Garden planet's population will not count towards the population listed on the Ranking page. If you pair an "increase population" satellite with the Green Garden planet, it will have no effect on your ranking!