Only one hand, AA.
KK would consider many factors position and no. of players, etc.
I need to respectfully disagree with this statement.
In my opinion, you need to start shoving strategically while you still have a stack large enough to do damage to other stacks. Players see that and are more apt to fold.
I assume you are speaking from a situation where you are really short-stacked and are about to burst that bubble, not-cashing. You simply cannot be afraid to be the bubble-boy (or girl). Everyone else at the table is fearing that, so take advantage of it.
I shove with any pair, A-9 and above. But again, it IS, AND MUST BE, a judgement call. Do not ever shove with trash....but, at the same time, do not wait for a monster hand or you will simply be blinded out, OR, someone else at the table will notice that you are scared to play and raise you constantly, knowing you will fold, sacrificing your ante/blind.
Also, being the first to shove has tremendous power. If someone before you has called/raised, you need a stronger hand to shove with overall. If it gets folded over to you, your shove has more of an impact. IF your stack size is enough to cripple the people after you should they lose after they call you, they WILL BE afraid to bust, and therefore, will fold most of the time.
Once in awhile, you will run into stronger hands than yours....someone will have AA, KK, QQ, AQ, AK, etc. There is simply nothing you can do about that...can't win them all.
Just remember: don't be afraid to bust. You want to get chips, and lots of them, if you want to survive deeper and deeper into the tourney after the bubble bursts. Do not be afraid to lose....the best players in the world bust out of tournaments. They are extremely high-variance.
Hope this helps.