I'm not sure how you could get to hundreds of properties unless you have an enormous income to fund them. On interest only lets say it takes 5 years of rent rises to send your property cashflow positive (that is being generous in today's market). Lets say you could only hold no more than 3 negatively geared property at one time due to income constraints. That means you could get 3 every 5 years at the absolutem maximum. So over 20 years that would be 12.
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A very niave statement indeed!
Have you never heard of properties that are cashflow positive from day 1. For example, I bought one property for $25k, which rented for $140pw. How many properties like that could you hold in your portfolio, even if paying P&I? The answer is unlimited. Properties like that usually don't have high CG, but they make up for it in yield. Obviously that particular property is no longer available, but there are still properties that yield a high return out there. You just need to look in the right places, know what to look for, or know how to change the use to make it work for you.
In the space of 3 years I bought 9 houses & a block of 4 units. They are not all regional properties like the above house, in fact 6 of the houses were Sydney properties. Granted they are not McMansions, & they needed a little tidying up. One of those is my PPOR & the others are all tenanted.
Do a search & read about Brenda Irwin. Go to the library & find one of Steve McKnight's books. Granted times have changed slightly, properties have increased in price, but that doesn't mean that what these people have achieved is not possible. Rents are moving upwards & it won't be long before those kind of deals will be widely available once again.