So, let's say the population of the United States is about 300,000,000.
And let's be generous and say that 99% of the population is infected by the zombie outbreak.
So then it's safe to assume that there are about 297 million people infected and 3 million survivors.
Now let's assume that Rick's group isn't the only group left out of the 3 million survivors. If each group has about 20 people in them, then that leaves us with with 150,000 groups.
And let's say that each person in a group kills, on average, 10 zombies a month.
This comes out to each group killing 200 zombies every month which means that each group kills 2,400 walkers in a year.
Which means that all these l'il Ricks, all 150,000 of them together, kill 360,000,000 zombies every year.
WHICH MEANS that after all the time that's passed in the show (about two years) the zombie apocalypse should be OVER BY NOW.