They'll keep making these as long as they make a good profit sort of like the F13th series.
Interest will probably wane after the sixth,but I reckon they will get 8 Saw movies done for sure.
I read somewhere that there are definatly going to be 7 Saws with the possibilty of Saw 8 before they end the franchise. Just tried finding it on the net but cant see it. Im sure there are going ot be at least 7 though.
The thing is, just because they decide to end the series, it doesn't mean someone else wont come along and pick it up again and drag it on. Like the Jaws films, Friday 13th Films etc.