Its not the lack of deals, I can tell you that - at least not for me.
The roadblock you have to getting to 50 houses is the financing and, in turn, the capital.
Not many people can afford to put down 30% of the purchase price plus pay rehab out of pocket for 50 houses. If you figure 70k purchase and 15k in rehab, you'd be talking 36k times 50 which is 1.8 mil. If someone had that kind of money already, they probably wouldn't be looking to get 50 sfh's. :-)
I've got 17 houses right now. For the most part, I've done that with very little of my own money. I was able to start with a HELOC on my home (40k) that I used on my first investment. I then realized how difficult it was to refi with the limited reserves I had left (10k) after putting down 25% plus rehab.
After spending quite a bit of time and finally getting it refi'd, i stuck all that back in reserves and haven't touched it sense. Thats my "bona fides" (yea, I will quote a john wayne movie every once in a while :-) for the banks to do refi's with me. I do end up putting in money here and there on deals. Typically, refi costs or maybe an overrun in rehab costs,etc. 3 to 5k per deal.
But the problem I have is getting loans. Small banks are still very gun shy about doing commercial loans on sfh's. They're convinced investors are the devil and, given the baths some of these banks took, I get it. I'm also out of conventional spots too. And, unfortunately, my wife doesn't qualify for mortgage (self employed).
So, right now, I'm kind of stuck. I'm in process of doing a blanket loan on my conventional loan properties to free up 8 or 9 more spots so I can reset my buying again. If I can get the thing to go through, I'll be able to get up to 25 houses at least.
But I recently had the same deal go through another lender that was going to do a blanket loan plus provide me a sizable line of credit. The loan officer said it was good to go, he vetted thru his VP, and everything was just about set for closing. At the last minute, the executive committee that reviews loans had one VP in the room that refused to budge and they put the kibosh on the loan.
So, to make a long story longer, the reasons why its so hard to get to 50 is financing, financing, and financing. If I could actually find a bank to give me an open LOC and lend me 70% LTV on my deals (appraisal value), I could have gotten to 50 somewhere around this year (10 houses a year instead of 3).
But since that simply doesn't exist for 99.9% of the population, I'm more than happy with the pace I've been able to maintain - even moreso given how little capital I had to start with.