As other's noted, the primary ways to gain more money are:Bring in partners for the equityTake on more debt, either through HELOCs, refi's, hard money or private lendersMake more money yourself.The most common way people scale is through the first two.  

I am based in the Seatle area, but I mainly invest in midwest markets and have scaled to 9 doors.

Now if you are talking building a whole developement of 180 or so homes , you have some leverage and economy of scale .

I would consider breaking even but will be hard to scale if you buy too many properties losing money

Not only is this prudent business management but if you're looking to scale down the line, having a creditworthy entity that can take business loans in its own name is a game-changer regarding risk allocation.

Happy to talk about the strategy more - we're up to over 20 rentals owned in WA now and wouldn't have been able to scale to that level quickly without househacking.

We want to scale up our STR business.

I was primarily interested in pooling resources with other rental property owners to take advantage of economies of scale and divide the cost proportionately among property owners. 

This decision alone can determine if you scale to 30 units in 3 years vs. 6 units in 3 years.

I've scaled to 9 doors now across Memphis and Detroit.