the six episodes was just a rumor. Apparently AT said it at a con, however she also said SHE got the info off the net. In his last chat, Joe said, to the best of his knowledge, no real contract with CN had been signed.
(his two year contract was with MGM, and just because he has it, does not mean he HAS to do the show. MGM could buy him out, basically pay him a predetermined amount of money. Sort of like he'll get paid $X if he works and $Y if they choose not to use him. of course, I have no idea his exact contract details, it's pure speculation)
And since only the first two eps of the season have been written (also as of that last chat) there is no definite number of eps for corin simply because they can't cast him until they know he's written in...and the scripts arent' written yet.
We could get a level of involvement like Hammond, a few scenes in every eps, or we could get like janet, who seems to be in 1/2 - 3/4 of them...or we could get like Jacob, one or two a year. (we already know that he's in the first two, so we get at least two, maybe three if he shows up for the finale.
Personally, I think by the second eps of season 7 he'll be neatly sent off to Kelowna where he's nice and out of the way. He may be trotted out once or twice, but we won't see him again until the series finale.
I really hope i'm wrong, only time will tell, and we won't know for sure for another year or so