Is there a limit to how many properties one can buy?

[Mike Brown]

Say you have $60k leftover every year to invest in rentals. Assuming 25% down, that's a couple homes valued at $120k a piece that one could invest in. I'm sure most banks would allow you to have 2 homes financed; however, this would add up in the longterm and I'm not sure banks would loan you any more money when you already have 15 or so rentals under finance. 

How does this work for those with plenty of money to invest? Do they just move on to bigger investments like commercial?