You can specify the the number of columns on a page through a layout XML file: using the layout
attribute of the page
tag.
XSD shema
Examples
Look at the examples from the core.
The default Magento 2 page layout has 3 columns:
The default frontend product view page layout has a single column:
The default frontend customer account page layout has two columns: a wide right and a tiny (sidebar) left:
How are the standard layout types defined
They are defined ion the layouts.xml
files.
Frontend layout types
Backend layout types
The common (both for the frontend and the backend) layout types
How to define a new layout type?
See the examples in the core’s unit tests:
https://github.com/magento/magento2/blob/2.0.0/lib/internal/Magento/Framework/View/Test/Unit/PageLayout/_files/layouts_one.xml#L8-L12
The official documentation
http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/layouts/layout-types.html