Add/insert a page in FM 12?

A one-off 'page break before' is a sloppy option adopted from word-processors, but available from the Special menu as well as by applying a local override on a paragraph style.

You can also define a style that includes 'page break before', and feel virtuous :-} Makes sense (the style, not the feeling) if you are working on a book that may require several occurrences of 'start section on a new page'.

Normally the text frames are set to autoconnect, so as you fill it with content FM will automatically add the appropriate left/right apges (depending upon your pagination settings).

You can manualy insert a page based upon a given Master page layout using the Special menu  "Add disconnected page" option.