honestly, i am having trouble answering that as a grammar-aware native speaker. We normally just say "in bulk." We normally don't modify "bulk" at all. "In bulk quantities" sounds the best to me, but "in bulk amounts" also sounds pretty close. "In bulk quantity" and "In bulk amount" both sound wrong to me. But again, i normally wouldn't say any of those, i would just say something like: "Hey, John, did you buy that toilet paper in bulk? You sure have a lot of it!"