Frenzy itself isn't an action or bonus action. Going into Rage is a bonus action and making it a Frenzy is part of the same bonus action. Since you already used your bonus action to start a rage you can't use it to attack. But I get what you are saying, going into a rage should be a free...
I have been working out the math for maximum undead from a RAW necromancer straight out of the PHB. The spell castings of Animate Dead below are considering the undead are already animated and the necromancer is just reasserting control.
At 20th Level:
3rd x 3 animate undead (4 each) = 12...