Limit maximum items in wp_nav_menu

I am working on a theme where it is unwanted to display more than 7 items in a wp_nav_menu (because more items would break this –somewhat non-standard– layout.

I can do this with css: li:nth-of-type(n+8){display: none;} (and I will), but is there a way to do this in php as well? It would be even better if this could be set in wp admin, where the user would be warned of he/she tries to put more items in the menu.

Every help would be very welcome ??