I agree with the others. I board horses to offset some of the expense of building the facility I need for my own riding goals.
I don’t think the under 5 or over 20 parameters makes sense. In reality, I’ve found that the magic number is however many you can handle on your own (mostly). When you start having regular employees, it becomes unfeasible until the business can support a FTE. That’s the part of it that does make sense…there’s definitely a donut hole there.
For me and for my dad who runs a boarding business in another state, that number is around 12. More than that and it is too much, especially given I have a professional job.
I also do a lot to create efficiency. Auto waterers everywhere, Nibble nets to reduce hay waste, etc. That stuff is an investment up front but it quickly pays off compared to hiring someone to do it the slow way. But you have to have the capital to invest in those improvements and it isn’t cheap.
I am an amateur and want to stay that way (ran a lesson business in my 20s, never again!) but to actually make a profit, I have a couple of sales horses. Sometimes that works out, sometimes it doesn’t but it is certainly my intent to make money on them! That reduces my boarder numbers. My personal horses are for sale for the right number at any time. My kids’ horse, no!