As the name suggests, I seek some clarification on this as googling didn't help me. I played a game two weeks ago against a Tzeentch player, and he used Rally on his deeply wounded unit of Horrors, though someone on the side told us that the number of dice for Rally is equal to the number of original models in the unit, meaning the 20 pinks, 20 dice in total that return a pink on each 6. Now, I can't for the life of me find this rule, as I'm about to have a rematch versus the guy, and he will bring one more unit of pinks for me to deal with. I looked at random threads, Tzeentch errata and the rule errata, found nothing. Does that mean, if they are outside of melee combat, they can roll upwards of 99 dice or more if even one final horror remains? Since 20 pinks are a total of 100 wounds, or more depending on what is returned. How do we keep track of how many died through the game as they split, get rallied through many phases, and keep splitting? I am pretty certain that those units of Horrors are impossible for the models I have to deal with, even with the rule being just 20 dice for the 20 starting pinks, and if the rule is made up, and the guy can roll 100+ dice, I'm throwing in the towel.

There's no clear answer at the moment. I would just have a discussion with your opponent and play by the "safer" rules, which is that you can only rally up to the starting number not higher. Rally and pinks is a hotly contested issue at the moment and is clearly in need of some clarification from an FAQ.

Sorry for the late answer, but... you roll a number of dices equal to the number of slain models in the unit. A unit of 20 models, with 14 slain models, you roll 14 dices. Certainly you don't roll for the number of wounds, and certainly not for the total number of models in the unit.