How investors got 100 doors using BRRRR strategy?

[Jack Ni]

I hear a lot of investors talking about Buy, Rehab, Rent, Refinance, Repeat to get as many as 100 doors. The number don't make sense and how do you refinance? Here is the math. 

 See if I am doing this right, Buy a house for $80k,  after rehab, it's worth $100k. It's rent's for $800 a month. Cash flow around $300(after all expense), If I can get 30 year loan with 5% interest.  Already have 2 house under the belt, so debt to income is already over 50%, how would I go about refinancing this home if I'm already reach my limit in debt to income ratio? If I add in rent income, it would not be enough (because rent income is about $300 and mortgage is about $500 per month (with tax and insurance)), it would increase debt to income ratio). 

Please enlighten me on this.