Something like calendar widget compared to a facebook widget with has constant updates
The more a widget updates the more battery resources it'll use. This is only natural... (using data/syncing will use juice)
You can check the amount of battery consumption by pressing Menu > settings > about phone > battery use. (if you think something is using excessive battery this is the place to check)
The more apps on the home screen might make a difference but I wouldn't think it would be anything noticeable. The display is gonna be the no. 1 battery consumer on your device either way.