UPDATE March 2017 : I have now expanded this functionality into an add-in for Visio (see Layer Manager add-in for Visio released ).
Visio layering system is more complex than any CAD system I’ve worked with, and often catches users out. A Visio shape can belong to no layer, one layer or multiple layers simultaneously, and a layer can be visible or invisible; printable or non-printable. In fact, you can have a shape on a layer that is invisible but printable – a good way to put a watermark across a document.
Firstly, Visio layers are per page in a document, and secondly, Visio layers are stored as an index number with an associated name. Therefore, layer 1 on page 1 could be named Outline, but layer 1 on page 2 could be called something completely different!
View > Layer Properties will show you the layer settings on the active Visio page, but the # column, which gives you a count of shapes on each layer, can be misleading because the same shape can be on multiple layers, and even sub-shapes (those inside a grouped shape) are counted.