Working on my first REI, a 4 plex. I'm using conventional loan. Im wondering if it's possible to get approved on a second, third, etc investment property, using conventional loan? Is there a limit?

The general consensus is that you can get 10 conventional loans under your name, but it can change based on the bank you use, your DTI, etc. Since you're working on your first deal, you should be good to keep going for a while before it becomes an issue. Usually, by the time you get close to the limit, you've found some better ways to finance your deals. Good luck!

The Fannie Mae limit for loans in your name via the Desktop Underwriter is 10, and the limit is 6 for manually underwritten loans. 

Thank you @Blake Maloney and @Bob Okenwa . I never thought they allow that many. DTI should include income from REI too right?

Most banks count 75% of your rental income as income to qualify for loans. So if you get $1,000 for rent per they will let you claim $750 of it as income.