Scalability and its secret sauce

[Dalton Steed]

First time real estate investor trying to understand the scalability of the process... I am looking to buy properties for the cashflow for now and the long-term asset to sell for retirement. I understand the buy and hold method, where you're just looking at the property as a long-term asset that would/could be used towards retirement. Where I can't see the light is how to create the "$10k" a month in passive income..? I've listened to the The Bigger Pockets podcast and have learned a TON from David and Brandon. Love the podcast and all of its content. But there is still something missing.. 

At an average of $200 per door, that's 50 doors per month. I know most will respond and say "you gotta do mult-family", which is the way I would like to go, but it still doesn't add up. Lenders are going to look at DTI and laugh when you say "I want 50 properties in the next five years". I have been self-employed since I got out of college (6-1/2 years). I've built and sold a business, made little money, and made lots. I still can't understand how it's realistic to expect to get to 50 doors in 5, 10 or 15 years. Say the average dollar amount to bring to the table is $50k ($200k house x 20% + Closing costs), even BRRRRing out of properties doesn't provide enough to buy 2-3 per year best case. I am just curious if there is some secret sauce that David and Brandon aren't sharing, or does everyone in here have debt out tShe wazoo..?

People love to say their net worth is "over a million", but I can't wrap my head around that. Most of those assets (properties) that give you that million dollar evaluation have liens against them. So I guess if you want to look at a Balance Sheet and you owe $200k on a property that is worth $300k, ok so you are $100k up, but that's assuming you liquidate that asset.. I look at things from a cash basses, not an accrual. Cash is real, 

I am probably going to ruffle some feathers, not my intention, but I would appreciate any feedback.