All the models in a unit of troops carry the same weapons, so it will be a unit of spearmen, or halberdiers, and so forth. It is acceptable to include the odd model which is differently armed for the sake of variety, but it is important that the overall appearance of the unit is not misleading. For example, if it is a unit of spearmen then most of the troops must carry spears. If a minority carry other weapons these are counted as spears regardless.

Many players like to mix models in a unit so that some are armed with swords, others spears, a few have double handed axes, and so on. This is particularly effective with Goblins and Skaven, where you wouldn't really expect much uniformity amongst the masses.

Mixed units may be attractive, but as far as the game is concerned they all count as being armed with swords, axes, clubs and other weapons which fall into the category of hand weapons. No special rules are applied to these units on account of their weaponry.