Depends on how much is sold. It could end on Inquistion or end up like the Final Fantasy franchise. I think this is the defining game in the franchise and will decide if more are made though. As long as they have consumer demand they'll continue to make them because they have plenty not explored yet.
The pessimistic side of me thinks this will be the last Dragon Age. I only say that based on my personal experiences with people I've talked to who were fans. It seems many of them think Bioware is on a heavy decline and have already made their mind up about DAI without playing it. I think this is sadly the mindset of many gamers. I guess it doesn't help this franchise hasn't seen any consistency in direction and all 3 games have been drastically different from each other. We all know EA will expect some unrealistic numbers for this game trying to capture a Skyrim market that's probably not realistic and umbrella parent company isn't going to be happy. I think it's obvious they're going for Skyrim's market with the way the game is being presented(especially the recent trailer). That combined with a holiday release and I don't even want to see EA's projections for this game. Budget couldn't be cheap for sitting so long on the project.
I just don't have much faith in EA these days. Hope they prove me wrong but their history speaks for itself. They aquire talented upcoming studios, milk them for every dollar until they don't have as much interest in their products anymore and then liquidize them. I'll enjoy it as it comes though and hope more keep being made.
I tend to agree that this game could be defining for the series. From everything we've seen so far I think that sounds like a very positive thing for the future of the franchise.
Also, with this coming to next gen I think it's well positioned to sell well, even to people who haven't played a game in the series before. With the Witcher pushed back and Elder Scrolls possibly being an expensive alternative...Dragon Age is gonna fill a hole. The only other RPG I can think of is that Bound by Flame game that is coming out in May... Add in some good promotion and I don't see any reason it would sell poorly, especially through the holidays.
And there really isn't a limit on how many games they make. For all we know Dragon Age 4 could cover the same time frame/overlap with Inquisiton but it could cover the events of the Anderfels and Tevinter or something like that.
While I agree all three games have been very different, I don't see that as nessecarily a bad thing. I think they have all had good and bad qualities, but being able to give players a variety of experiences within the world of Dragon Age is ultimately a strength of the series. I still believe that if they had had the resources to make Kirkwall an openworld city, giving you the ability to actually explore etc, it would have drastically changed the argument about 'containing' the player in a fantasy setting. (And could have made DA2 a very memorable and unique gaming experience).