i am in a similar boat as you with 12 units and 2 garages. The break is (4) 2 unit town homes and (4) 3/2 rowhomes. I bought all mine cheap and fixed them up to be nice long term rentals that could be resold if i needed to bail out for some reason. i make a small amount of money on the single units but the 2 unit are cash flowing much better and i am selling all my singles. I would recommend you find properties that are already 2 units that need work and fix them up and rent. As an example i gross over $2050 on one 2 unit that is a 2/1 in each unit. One unit rented to section 8 for $1150 and the other to market tenant for $900. Could be even better if i rented both section 8. My best 3/2 is rented for $1450 to section 8 tenant. That is a $600 a month differential for what is essentially 1 more bedroom and another kitchen. In the long run the 2 units will out cashflow any single family and allow you to paydown your loans if thats what you want to do. It would be even better with 3-4 unit properties if you can buy them right.
You should also look into section 8 tenants. They arent all bad and i was a big skeptic until i dipped my toe into the pond. They need to be managed like any other tenant but i get a premium rent of $200-400 a month per unit over market tenants when i rent section 8 and to me this is worth the additional hassle.
Bottom line is ditch the ones not making money and buy something that makes sense from day 1 on a 20-30 year mtg so you can cashflow better and allow you to buy more and build your capital base. Read all the articles from Jeff Brown on this site. He explains things on another level that makes sense if your in for the long game. If you dont need the income from your rentals for 10 - 15 yrs, his plans may be for you.