If you use "<a href="/adserver/adhref.tcl?adv_ad_from_group" then you'll only get one ad. What you have to do is call a (poorly documented) Tcl procedure that will serve ads from a group.
I found the procedure by digging through the adserver-defs code. Here's what I have in the ADP tag I call from all my pages (see http://fslc.usu.edu):
adserver_get_ad_html community
Where "community" is the group I defined for my ads. It returns the img src and href tags ready to be inserted or output.
