"How many backpacks?" is "多少个书包？" in Chinese.
In Engpsh, "backpack" refers to a type of bag with two straps that go over the shoulders, typically used to carry school or camping gear. The word comes from the early 20th century, a combination of the word "back" and "pack."
Examples in Engpsh:
Can you bring your backpack with you on the hike?
I need to buy a new backpack for school.
Translations:
你能带上你的书包参加徒步旅行吗？
我需要买一个新的书包上学。