normal magazine for a 6906 holds 12 rounds. The 59 series mags will fit as well. They were 14/15 rds, depending on the factory follower, which you can swap out. Now for aftermarket mags, the best ones are MecGar, which may have made the original mags for S&W anyway, depending on the year and model. Their quality is top notch.
MecGar makes 15 rd flush fit, and some 17 rd flush fits as well. The 17 rd mags have a funky tapered spring that sort of accordions inside itself, and have a hollow and shallow follower. They also make a 20 rd magazine that will stick out even more.
In my opinion, I would only get S&W mags, or MecGar.
There are some 30 and 32 round super long mags, but the quality is iffy. S&W did make some pretty rare 24 or 25 rd magazines. Never seen one in person though.
I get my MecGar mags from CDNN, which has $6 shipping this weekend I believe.
http://www.cdnninvestments.com/smithwesson.html
I have both a 6906 and a 5906 (as well as a KelTec P-11, which can use S&W mags)