> it is possible to increase I number of clients vpn:pptp > 16 ???

Yes,
You will have to extract the binary image, edit the line in
/etc/inc/globals.inc in the $g array that reads "n_pptp_units" => 16,"
to however many connections you wish to allow.  After you have edited
that you must rebuild the image, and then upgrade your firmware.

I posted a script to unpack/repack images awhile back.  It's located at
http://mrkasai.undergroup.com/m0n0wall/ called m0n0image.php.  I have
had success in using it, but others have reported problems.  

I quick edited this and recompiled a generic-pc and net45xx version for
50 connectons.  I would warn against using these on anything you can't
get to and reflash as I cannot promise a successful upgrade.  However,
if this is a test system hopefully this will save you some time if it
works.

images are at:
http://sio.midco.net/storage4/