Well, considering they're always getting hurt for extended amount of time, maybe eight if you're playing a 4-3 defense?

So I am fairly new at this game myself, but here is the most effective roster breakup I have found so far (from personal experience with a little advice from some helpful veterans)

I start with looking at the 46 active roster spots I can use. If you have a P and K (highly recommended) then you have 44 left. Being a little OCD, I split that up for 22 on offense and 22 on defense. From there it depends a little bit on your style of play, but this is what I have been using and trying to fill my rosters up with.

Offense- 2 QB, 3 RB, 1 FB, 3 TE, 5 WR, 8 OL

Defense- 4 DE, 4 DT, 6 LB, 5 CB, 3 S (If you really want a 5th DT, take it from the DE or LB position, but your backup DEs and LBs make for good special teamers)

Then you have 7 extra roster spots to do with whatever you want. I do like to have about 2 extra OL, 2 extra DL, and at least 1 more DB.

I know this is a little broader of an answer to your original question. But my take is that if you have 8 DTs on your roster, you're probably undermanned at another position, and may want to weigh those risks. Having 6 (or 7 in extreme cases) should be more than enough.

I would not actually count DT and DE separately most of the time in this game. Their skillsets are remarkably similar, and the amount of injuries means that you will often have to play DT at DE and the reverse.

That said, you probably want to focus more on acquiring more DE and have the best run-stuffers backup on the inside as well.

I personally like to look at real NFL rosters. This will change if you are 3-4 or 4-3, so find an analogue in the NFL and see how many total DLs and total LBs those teams typically roster, but it is pretty typical to have players shift around during the season, so focus on the total numbers, and not how many DEs or DTs specifically.

As a 4-3 team I went with four. After one regular season game two of the four are already nursing injuries. Maybe I should've went with 5 or 6.

Its always good to have a few hybrid players that can slide into other positions during the season.. obviously DT, DE can do this as well as S, CB... TE OL RB... I like a 4-3 and I usually keep 10 players DT,DE

The injuries are to frequent and it's not very clear when players are ok to play with injuries. This is a "game" and although I love the fact injuries occur to mix things up.. the game still has to be enjoyable, it doesn't need to simulate real life in this area. My only wish is something to be done about the domino effect in position groups