Q: what is the volume limit code for ipod nano ?

That's something the user chooses, when a volume limit is set.  This is so a parent can set a volume limit for a young family member (that's not changeable without permission), so he/she doesn't blow out ears before age 20. 

If you're the "young family member," you should probably keep the indented volume limit.

Otherwise, if you don't know the code, you'll need to do a Restore on iPod in iTunes to erase the iPod and put it back to "factory" settings.