Hi

I've read of successful m0n0wall people who manages to interconnect two 
networks to a central location.
Anyway, since you might require the >16 VPN clients per m0n0 box, this 
url will help you to achieve that.
http://www.michael-i.com/files/projects/m0n0ad/index.php


>We may have the opportunity to set up
>a medium size VPN network in the near future.
>We're looking at about 80 clients all
>interconnected from a central location
>over ADSL and Cable links.
>