There are now 53 in all. It looks like they haven't been fully set up yet but it looks like they are all medic related.

Check out your personal achievements in Steam Community. If you can't find it, click on My TF2 Stats link below.

Hmm that looks strange. More like a bug though..
Why should there be so many and only for the medic class.

EDIT: They are now all gone again, at least for me.

because they think not enough people play as medic. that's the only reason i can think of.

Confirmed by Valve. What I saw was probably them testing it or accidentally putting them on the live server, rather than the test server.

Linky

Wow awesome that they are adding achievements for all the games we already played through too..
Now I can play them through again without feeling like I'm wasting time..
Oh wait!

I can already imagine the fun battles we'll have after the first achievement pack goes live. Full teams of only medics battling eachother

On a serious note I do think they used the achievements in Portal very well.. Stuff like "Assume the party submission position" popping up at you.
Oh and the achievement in Episode 2 where you have to launch the garden gnome into space was hilarious, although I couldn't bare completing it myself.

cool