I encrypted my nexus, and now forget the password after the power is run out Can anyone tell me how many digit required and its rules (upper case, number, etc)?
According to an enlightening blog post by Nikolay Elenkov the master password is derived from your screen lock password or PIN. These screen lock passwords are limited to 16 characters but don't have any rules by default (unless you have a device administration policy enabled that enforces something different). Assuming you haven't done anything different than the normal encryption process, these should be the password restrictions you're bound by.
As an aside, according to Nikolay, there is no character limit or rules on the master encryption password fundamentally.