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> I saw that pptp tunnels are limited to 16 tunnels max. Is there a good 
> reason why it is limited to 16 tunnels ? Due this is because of mpd ? or 
> just for avoid to eat all memory ?

I think to keep m0n0wall usable on low-memory/low-power hardware...

> Another question is how to raise this limit to a higher value ?

Simply fetch Michael's (good) document at:
 
http://www.michael-i.com/files/presentations/getting_started_m0n0wall_development_v0.1.0_050604.pdf

There is an example (on page 8) which shows you exactly what you need:

"We are also going to expand the PPTP user limit from 16 to 50 and, 
consequently, the PPTP subnet mask."

I hope this helps you,

-- Vincent