I always had SFR and duplex's previously-and still have 2 duplexes.Having 12 units under one roof gives you much better economies of scale-I increased the rents and improved the units as tenants moved out and now have the confidence and experience to tackle 30 units at this point.

We are aiming to scale higher, and are now actively pursuing smaller apartment buildings (20-50 units). 

@Mike LattierLike others mentioned, I think the younger you are, you have the option to take more risk.Im 29 and my mindset is I can lose it all before 35 and still make it back much quicker the 2nd time (doomsday scenario)I think there is a sliding scale of risk when it comes to any type of investing and in real estate you have different markets, price points, neighborhoods, types (sfh, multi, mix use, storage, etc) that may all factor in differently with your goals.For example, I invest heavy in C class right now for class flow but if I was older id transition many of these into B class

Something to consider when you scale up.There are a couple of CPA's that specialize in real estate taxation on this forum.

Economies of scale are scant with only a door or two; but singles and doubles are generally easier to buy and sell.

Even on the smaller scale, I would only use the 50% rule as a quick tool to see if its worth underwriting further.

Though I will still run all my numbers using subcontractor bids because the plan is to scale to a point where I can build a flipping business. 

mom and pop coffee shops / roasters don't scale well as business - that's why you don't see anything between 1 or 2 stores and starbucks / peets even in CA. you need to find an entrepreneur passionate about starting a roasting company, rather than finding an existing roasting entrepreneur and try convincing them to open a new branchwine bar / beer bar are easier, but depend on immediate demographics in the area and trends

Now the value has gone up by 80k and I used the heloc of build in equity from only leveraging 5% down to scale my rental portfolio to 18 units.

It's really a matter of economy of scale.