if its specific dudes that matter look at the alive command (combined with not, perhaps, as in "not alive)
if its numbers look into the "count" command. you can use it on groups, west soldiers ina specific area via thislist in a trigger, or use logic to add up the numbers inseveral groups or whatever you like
(count units group1) + (count units group2) < 9
or whatever, check the syntax, spelling and everything else. you can use this in the condition field of a trigger or in a script, which you will probably want to loop. If you're not sure about loops it came up in this forum a couple of days ago or check Johan Gustafsson's excellent scripting tutorial. The whole count thing came up the other day too, so try a forum search.