What is the rule for nice answer badges? I have 2 answers with 10 upvotes, but I only received only one nice answer badge.
Nice answer badge is awarded (sometimes there is a delay) when a question receives a score of +10.
It should be noted that if you receive a Nice Answer badge, you will not lose the badge even if the answer happens to drop below +10 sometime in the future. BUT, if it did drop down and another item goes to +10, you will not receive a new badge for that item (your badge count will remain the same because it will go back to matching you current totals).
The badges can take a little while (usually of the order of minutes) to come through. It all depends on the timing of when the procedures are run that calculate what badge criteria have been met.
Jeff could tell you how frequently they are run, but I suspect that he would like to keep it quiet.