Ok, numbers; I think the key is the combination of high number of bedrooms with full/queen beds and low cost to purchase. I own one that is located in south Philly and it brings about 3k consistently, I spent 120k total between purchase and rehab. It has been running for about 9 months. I just opened a second one in the suburbs of philly, delaware county, where my purchase and rehab is about 65k. Furniture is not included on the purchase price, but give an take about 10k for all of it. Mostly ikea stuff. The other one is on a less desirable location, but it is working, bringing is about 2.5k for the last two months. I just opened it in the middle of march.
The biggest cost of running an airbnb is the cleaning cost. So having one isnt the best, so I'd say three is the sweet spot where you can have a cleaning person to turn your units financially efficient. I use to charge 35 bucks per cleaning, but I upped to 45 because I was struggling to find a cleaner.
My cleaner also does the laundry at her own place, because she doesnt want to wait for the three loads of laundry to finish.
If you want to see my listings I can pm the link.