7
How about this: since it's death note, let's base the rules on names. Somebody posts the number as well as a temper's name (in spoiler tags) and description of the temper (it can be a lie). If somebody else posts and his/her name is in the previous post's spoiler, he/she gets exempted for the round. Since the description can be a lie, you have to decipher whether it's a lie or not, which adds this tactical death note taste. Once somebody is able to reach 10 without being exempted for the round, the tenth poster can chose somebody (who posted during the round) to kill. The one who was killed will be exempted from the next round (which means that you won't be exempted the 2nd round because you didn't participate in the 2nd round). The only problem would be cheating by looking at the spoiler beforehand...
EDIT: also, the previous post can be edited by whoever posted it to the name of the next temper. If we were to use this rule, an edited post can't be qualified.