I would imagine that your starting capital requirement would depend very much on your target market. Assuming you're looking at "distressed" sellers (i.e. those willing to take a hit on the asking price in return for a fast sale), you'll need significantly less than if you're buying full price and long term.
Things to bear in mind are that cities often require 2-3 times as much capital per property (in the UK, you can start with about £35k outside London but need about £100k inside the city), that your acquisition rate will depend very much on how quickly you can remortgage and that you absolutely MUST make sure your cash flow covers everything needed to run your properties.
Far too many people get burned because they underestimate how expensive it is to maintain a property portfolio: you're better off not risking so much and building a bit slower.
First thing to do is to check your country's remortgaging rules. Then look into how many mortgages your bank will let you have. Then start looking for short-term investors from whom you can borrow the initial deposit and pay back over the first 6-12 months (from your tenants' rent).
There's a LOT of research and due diligence to do with real estate. Don't get into it lightly. (But yes, it's an awesome investment business!)
Hope it helps.