Read the spell Animate Dead.
Because a limitation like the one you mention doesn't exist unless it appears in the spell's description, which it does not. I assume that what you are thinking of is Animate Dead in D&D 3/3.5, insteead of Pathfinder.
The table in the Skeleton template that gives CR and XP to defeat by hit dice only goes up to 20 hit dice -- but I think that just means that you are on your own to work out those stats for skeletons with more than 20 hit dice.
Makes sense.
Hit Dice: A skeleton drops any HD gained from class levels and changes racial HD to d8s. Creatures without racial HD are treated as if they have 1 racial HD. If the creature has more than 20 Hit Dice, it can't be made into a skeleton by the animate dead spell. A skeleton uses its Cha modifier (instead of its Con modifier) to determine bonus hit points.
There's the quote you're looking for (emphasis mine).
The fact that the table stops at 20 is immaterial. There is no rule stating that the template cannot be applied to creatures with more than 20 HD, and any other method of creating skeletons could still do so.