Well I'm having trouble with this feat on it's wording. I assume by what it says is that the command undead ability has no bearing on how much HD worth of undead you can control? Rather just says it doubles the time.
As for the animate dead so... 2 x Level + 4 (undead mastery)
So a lvl 6 wizard casting animate dead is able to animate 20 HD worth of undead.
What about controlling the undead you just animated... hmmm?
Honestly blowing 1 feat just to get undead mastery is not really worth it.
There are two things called 'Command Undead':
There is a feat called Command Undead, that allows you to use Channel Negative Energy to command any undead withing 30 feet of you. Normally you can control a total of (Cleric level) HD of undead. With Undead Master, that limit is increased to (Cleric Level + 4) HD.
There is also a spell called Command Undead, that targets only a single undead for 1 day/level. With Undead Master, the duration you can control this single creature is 2 days/level.