The Statistics tab displays the following information about the assembly structure:

Local sub-assemblies the number of local sub-assemblies;
External sub-assemblies the number of external reference sub-assemblies;
Local components the number of local components;
External components the number of (loaded) external reference components;
Unloaded components the number of unloaded external reference components;
Total the total number of elements in the assembly.

Take a look at the video list  e-Learning on Assemblies and Mating