We started in 2015 with a 4-plex. The rents from those 4 units have now paid off the purchase price of the building + the extensive renovations we did as they became vacant. We got all of our money back with this months rent! Thank you tenants!
That money will now go into a separate account & will pay insurance, taxes plus some "maintenance money" for all of our properties. We have 10 now - 11 if we include a small office building we bought. Most of these will be totally paid back by our tenants within 5-7 years of purchase. That said, we paid cash for each - so no mortgages. Still, I wouldn't have paid additional on the principal with rent money - as others have said, keep it for unforeseen expenses and for future purchases.
That one little office building gets us nothing, we bought it when we retired & closed our Dallas business & needed a transition place for all that stuff. Without it we'd have had to pay storage somewhere. Now, we use it for left over materials & when we buy sale appliances, closeout stuff, etc. (to keep on hand should an appliance go down so we don't have to rush out & pay retail) and store it there. We are setting up a small office over there for us to get all of our paperwork, etc. out of the house. We are also considering putting in a small air b-n-b type unit in part of it. Not that our small town has a big need, but it would also serve as a guest room for us on overflow holidays. We recently found out zoning will allow us to put a small house on the back part of the property. That is a consideration for the future. So you never know what fun kind of things are going to present themselves.
My point of this rambling post is, I guess: save your profits & keep investing. Good luck!