There were 68,650 Mustangs produced in 2011 (26,422 GT, 5,026 Shelby GT500, 37,202 V6)
I would expect similar numbers for 2012 when it's all said and done. That's a lot. I've seen the gearbox gripes and complaints but they've been few and far between. The ones that show symptoms appear to be pretty persistent problems. All in all though... I think there have been relatively few problems. Time will tell the real story, I suppose. I think back to the mid 80's and all the folks that were blowing out 3rd gear on the T5 or chewing up TOBs on the aluminum bearing retainer.