I doubt they're making "millions" selling brand new sites. More realistically think of it like farming, and selling the sites once they've reached a minimum maturity. This is going to be a long scrolling response. It is to visually see what I would do, if it were me. I'll use the beginning of the year as an example. And most likely, it will be automated, because it will get hectic when the plate gets more and more full. Keep in mind, this doesn't take into effect the ACTUAL LABOR INVOLVED with doing linking, writing content, etc. Hopefully you have a decent team, otherwise you wont be able to sustain the cycle.
January:
Start Site 1
Nurture Site 1
February:
Start Site 2
Nurture Site 2
Grow Site 1
March:
Start Site 3
Nurture Site 3
Grow Site 2
Grow Site 1
April:
Start Site 4
Nurture Site 4
Grow Site 3
Grow Site 2
Grow Site 1
May:
Start Site 5
Nurture Site 5
Grow Site 4
Grow Site 3
Grow Site 2
Grow Site 1
June:
Start Site 6
Nurture Site 6
Grow Site 5
Grow Site 4
Grow Site 3
Grow Site 2
Grow Site 1
July:
Start Site 7
Nurture Site 7
Grow Site 6
Grow Site 5
Grow Site 4
Grow Site 3
Grow Site 2
Grow Site 1
August:
Start Site 8
Nurture Site 8
Grow Site 7
Grow Site 6
Grow Site 5
Grow Site 4
Grow Site 3
Grow Site 2
Grow Site 1
September:
Start Site 9
Nurture Site 9
Grow Site 8
Grow Site 7
Grow Site 6
Grow Site 5
Grow Site 4
Grow Site 3
Grow Site 2
Grow Site 1
October:
Start Site 10
Nurture Site 10
Grow Site 9
Grow Site 8
Grow Site 7
Grow Site 6
Grow Site 5
Grow Site 4
Grow Site 3
Grow Site 2
Grow Site 1
November:
Start Site 11
Nurture Site 11
Grow Site 10
Grow Site 9
Grow Site 8
Grow Site 7
Grow Site 6
Grow Site 5
Grow Site 4
Grow Site 3
Grow Site 2
Grow Site 1
December:
Start Site 12
Nurture Site 12
Grow Site 11
Grow Site 10
Grow Site 9
Grow Site 8
Grow Site 7
Grow Site 6
Grow Site 5
Grow Site 4
Grow Site 3
Grow Site 2
Grow Site 1
Now you have a year of traffic on the first site. You can see it's traffic has continued to grow. You can see it's ebbs and flows. You can sell it for it's STRENGTHS, while having a better predictability of it's weaknesses. Hopefully they were some sort of money sites, so you can have a predictable "worth," and make your money during the growth as well as the sale!
Anyway, to continue the cycle...
January:
Start Site 13
Nurture Site 13
Sell Site 1
Grow Site 12
Grow Site 11
Grow Site 10
Grow Site 9
Grow Site 8
Grow Site 7
Grow Site 6
Grow Site 5
Grow Site 4
Grow Site 3
Grow Site 2