No and yes. A trigger is not "per unit" or "restricted to 1 unit". It is something that runs on the entire map and that executes its "Effects" when its "Conditions" are met. There can be as many triggers on a map as you want. They CAN of course affect the units on the map.
As for a site,... I think this site is one of the best sites you can find to learn scenario design. Check out the scenario design section on the main page, search for guides in this forum, or download one of the multiple guides in the downloads section.
Also, for these kind of questions, please use the question thread (top of the page) in the future. It helps to keep the forum nice and tidy
[This message has been edited by angryzor (edited 12-19-2007 @ 06:31 PM).]