Each discipline has its own legend, usually on its first sheet. Some plan sheets carry a small legend for that sheet only, such as a wall type legend.
Example: a wall type legend shows one hatch for a rated wall and another for a standard partition. A drywall estimator measures each hatch as a separate quantity.
Read the legend before you count. The same symbol can mean different things on different projects. A legend is often a standard list, so it may show symbols this job does not use. Count what is drawn on the plans, not what the legend shows.