The basic use of this spell is to duplicate any other spell of 8th level or lower. You don't need to meet any requirements in that spell, including costly Components. The spell simply takes effect.
If I choose to duplicate Magic Missile do I create 3 darts (level 1), 10 darts (level 8), or 11 darts (level 9)? Or can I choose how many to create from some range?
In other words, what is the level of the spell slot "used" by the duplicated spell?