[Groff] Number variables in mm

[lxnf98mm]

I am just starting to use mm
How do I set the number variables
For example

 N      Numbering style [0:5], default 0.
        0 == (default) normal header for all pages.
        1 == header replaces footer on first page, header is empty.
        2 == page header is removed on the first page.
        3 == "section-page" numbering enabled.
        4 == page header is removed on the first page.
        5 == "section-page" and "section-figure" numbering enabled. See also 
the number-register Sectf and Sectp.

Using ".nr N 2" does nothing that I can tell

Richard