Some 'Gods'...

Eric Harris and Dylan Klebold both suffered from the same superiority complexes as these two characters did. Harris even referred to himself as a living god many times in his writings. They did use pipe bombs and homemade explosives, and only killed 15 people, including themselves.

However, had the two 20-lb propane bombs they planted in the cafeteria gone off as planned, the death toll would have been as high as 400. Investigators determined the bombs were powerful enough to kill every student in cafeteria, and the library would have collapsed onto the commons, more than likely killing all 56 students in it at the time.

Unlike Harris and Klebold, however, I don't think Andre and Cal had a specific "plan." they just wanted to go into the school and take out as many people as they could. Harris and Klebold had planned to shoot survivors of the explosion (see above) as they fled the school, from their pre-determined positions in the parking lot.

Killing people is hard and you'll notice these aren't trained soldiers, they're a couple of highschool kids with limited shooting experience. Also the movie didn't mention how many were wounded.

They also aren't very efficient, taking time to torture their victims. I don't think it was purely about the numbers for them.