As was mentioned above the actual limit is 10...and you can go higher in certain scenarios.  

If co-signing but NOT occupying, then you will need to put down 25% down-payment as @Andrew Postell mentioned. 

Like @Marcia Maynard mentioned, does it cash flow?

Shortly after submitting my question, I went to the Calculators page and saw where it mentioned that the download option was for "Pro" users only.

They haven't been determined yet because the costs of the items Tom Gimer mentioned are put together at the end of the year and reevaluated every year.

As others mentioned, it depends on what you define as 'investing'.

I found the following article to be interesting:https://www.cnbc.com/2017/11/06/homelessness-soars...Nobody wants homeless people in their neighborhoods yet those very people buying the houses in today's market are in a way, contributing to the growing population of homelessness (in the markets mentioned in the article of course). 

Additional upside can be found in the areas you mentioned - ease of access, economy of scale with service providers, time efficiency for management, possibly offering additional price points in your portfolio, ...Contiguous property is worth more to adjacent owners.

However, this experiment in crowd-sourcing has resulted in a sort of local semi-celebrity status, with numerous mentions in area magazines, blogs, and whatnot.

@Sean Walton mentioned keeping other buyers lined up in case the first buyer falls through.