Generally a 4-3 defense will keep 4 DT's, and sometimes even 5. Historically, John Fox has generally only kept 3 DT's on the active roster, opting to keep a 5th DE that can play some tackle(Ayers most likely). I hear/see a lot of people that are expecting more on the roster, but don't expect it.
I also think that's why we went with stop gaps this year, as if we draft a high pick, he'd be slotted in one of the 3 roster spots Fox designates for the position.
Currently competing for the 3 spots are Marcus Thomas, Kevin Vickerson, Jeremy Jarmon, Louis Leonard, Mitch Urien, Ryan McBean, a couple UDFA's, and the newly acquired Broderick Bunkley. Who are you're 3?
Right now I'd say Vickerson, and Thomas likely start with Broderick Bunkley being the main backup, it's really only between him and Jarmon in my opinion, but it's gonna be a tight battle given how few guys are kept.