This is a known glitch. Lots of people have this problem but I think it's being looked at so hopefully it will be resolved soon. You should consider yourself lucky, though; mine are stuck at 356...
It says I have 12 notifications but I only have 2 notifications available... Why is this happening?
Just a glitch, sometimes it says I have 8 but its only 3. my number isn't stuck like some are complaining of.