To play Pitched Battles, you have to have a spesific amount of unit categories based on the pointscale bracket you are in.
The categories are: Leader, Battleline, Behemoth, Artillery, Other.
The infomation about a units keyword are at the bottom of the warscrol,l the warscroll is in the Battletome or Grand Alliance books.
The points value, number of models per unit, and category limits are all info that is found in the Generals Handbook. (unless you collect an army that was released after the annual generals handbook was released, then the info is in your battletome. a new edition of the generals handbook is released every summer)