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 whilst continuing to count the whole unit as identically armed, 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 and where a few 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. As far as the game is concerned a mixed unit is treated as if it were armed with hand weapons (axes, clubs, maces, swords and such like) even where it includes a mixture of other weapons. No special rules are applied to these units on account of their varied weaponry.
All troops are assumed to carry a sword, long dagger, or other comparable hand weapon. In addition, some troops carry another weapon such as a spear, double- handed axe or halberd. Troops armed in this way can choose to use their hand weapons rather than their other weapons if the player prefers, but must then continue to use hand weapons for the entire duration of the combat. For example, troops with double-handed axes might put them aside and fight with swords where they do not wish to strike last.