Do you hope to eventually be in a position to live off the passive income from buy-and-hold investments, or just work a standard W-2 job and use REI as supplemental income?

When it gets built up, you can draw on it tax free to supplement retirement. if you die early, you leave it to your family.

There's so much I don't know, but it's awesome.Right now my real estate goals are just to supplement my current income, but in the next 2-5 years I'd like to be able to use real estate investing as my full time income.

With my duplex that has a garage, I offer the resident a reduced rate (but it's still supplemental income for the property) but if not I'd rent them out separately given that it makes sense for the area (i.e. might not work in an extremely rural area). 

Now I would like to complement the idea of moving there with a rental property that can supplement my income.  

Hi all,I'm interested in acquiring a small (~10) portfolio of SFH rentals to supplement my income in ~15-20 years.

I also own a cabin up in the mountains north of Phoenix and am considering doing short-term AirBNB rentals with it.My goals are to build up a small portfolio of multi-family units in the Phoenix area to supplement my income.

Best to identify exactly what you are hoping to achieve - supplement or replace income, how much, etc. 

Over the years I have always supplemented my income even when I made a very low day job salary at a full time job.

This will avoid the risk of losing a tenant over a rent increase and regardless of how good your tenant is it is poor business practice to supplement a tenants rent.It is not good business to not raise the rent as not only are you supplementing a tenants rent you are also devaluing the resale of your property.Research your local rental market adds and put your rent at market for the size and location of your property.