We've seen this before, but we solved it back then too. I guess I'll do it your way since it's your thread.

This should make it obvious!

not really
121 is 22

22 is 10

10 is 3

3 is 5

5 is 4

4 is the magic number

1,000,000,000,000 is 12

12 is 6

6 is 3

3 is 5

5 is 4

4 is the magic number

I tried to start with a high number but it didn't seem to help. I was going to do 123,456,789, but it got kinda complicated on what exactly that would be.