Not that there is an actual number and LE will go after low-hanging fruit who may even have small quantity, but as far as importing from big vendors overseas in large quantities, what do you think is the smallest amount they would go after someone for? An example of this is they find 3 different MDMA vendor packs at customs, one is 10 g, the other is 50 g, and the other is 100 g. Are they going to go after a 10 g pack or even a 50 g one?
I can speak from personal experience when local LE does find some MDMA it is fucking level 5 code red serious. They don't even really understand the chemical but it will fuck your world in court.