No the Zombies start out at 15 strong (largest single unit on the board) and only in one horde. As Necromancers raise more dead, any zombies created will increase this horde. Any enemy killed and infected by the Zombies will also immediately add to the pile of this shambling horde, but only on a D6 roll of 4+. So, potentially the horde can get massive, so there is a need for all the heads.
The reason they all stay together is to stop powergaming small bands of growing undead dotted all over the place and gaining too unfair an advantage. Another advantage would be to keep these chaps back until the horde is awesomely huge HOWEVER, they must engage as soon as they can and must move their full movement each turn towards the enemy. I've made it this way on purpose and besides the Necromancer is just there to keep them up and active and from roaming off to the mall (..why do zombies always go to malls?!.) Anyway, he's got enough to do without tricky stuff like that.
@Karetsu: All Wights and Wight Kings (new Cavalry Unit) have the eyes thing. Warhead may still get flaming eyes (good call), but only when he's raging!