Beginners usually forget to apply map locking options and often create every map from scratch. As a result, they are overloaded with multiple map composers that have permanently changing maps. If you want to save your time, then it is wise to use Lock layers for map item and Lock layer styles for map item to block undesirable changes in your maps.

You can have multiple maps and atlases that are stored within a single project, and get access to them by going to Project | Print Composers. Advanced options are available at Print Composer Manager, which is under Project.