Scorpions will sting mainly for when they're hunting or for self defense. They can control the amount of venom they use accordingly.
Scorpions use their stinger for two main reasons. One, they're hunting. They use the stinger and the venom to subdue their prey so there is no struggle when they eat it. Or two, they're being threatened. They can actually control the amount of venom they are injecting into that object depending on whether they are using it for food or for defense.