The number of layers you need on a page, depends on the purpose. The default is one layer per page.

Concurrent work of one editorial designer and advertisement staff on the same page

If you want to make possible that editorial designer continues to work while advertisement staff lays down the ads on the same page, you need at least two layers: one for editorial, and one for advertisement.

Concurrent work of two editorial designers and advertisement on the same spread

If you want that two designers work on the same spread, you need two editorial layers: one for left side of the spread, and another for the right side of the spread. Together with one advertisement layer, it makes three layers.

National and regional ads

You need more than one advertisement layers: two for editorial, and one for advertisement.

Alternate design ideas

If you use layers to display alternate design ideas for the same layout, in such case, you need as many editorial layers as ideas.

Improving performances

Another use of layers is, for example, if your page prints slowly because it contains many large graphics, you can use one layer for just the text in your page; then, when it’s time to proofread the text, you can hide all other layers and quickly print the text layer only.

In such case, you need at least two layers: one for text, one for graphics