how many units till i reach retirement

[Dustan Marshall]

I am always thinking of the long term.  I only have a few units now but my plan is to buy one unit every year (by doing a cash out Refi from the previous year).  The idea is to purchase enough units to suppliment a comfortable income.  As such in looking over the number I wanted to get everyone opinions.

20 units @ 700/mo rents (average) $14,000 x .40 (60% to cover taxes, insurance, vacancy, repairs, PM and lease up) = 5,600 gross monthly income.  

Then $5,600 x .75 ( 25% tax bracket) = $4,200/mo net income.  To cover health care, vehicle, primary home costs, groceries, daily expenses.....etc

These numbers assume all houses are paid in full with no debt services.

Are my numbers to conservative?  Not conservative enough?  I am missing something?