I have to purchase 1 more property, but I only have 2 slots left on my list.

I don't have four residential loan slots left.

You can have 10 conventional mortgages in your name, a spouse can have 10 in their name. 20 mortgage slots available for a married couple.

I am in the process of considering property management software and wanting to hear from others how they got a good majority of their tenants to switch to paying rent online instead of writing checks.As we have rental offices at our properties, most tenants just drop off handwritten checks there (we even have a mail-slot for after hours), so there isn't that much work involved.  

That is unless you make enough cash flow to hire someone to empty those coin slots.

The new Seattle ADU/DADU legislation is real game changer.

The appraiser will look at the additional bedroom / bathroom counts in the DADU, then add them to the SFR bedroom/bathroom counts already on the lot. 

Once you get 10 on the books, then roll the majority of those to a commercial blanket loan, and open up your 10 Fannie Mae slots again.

I'm guess the upside is vacant slots infill, and about half your expenses were labor (guessing that's the owners take).

You can each have up to 10 loans, so if there are 4 of you, you could have potentially 40 properties collectively before being out of Fannie Mae slots.