can "많이" be omitted from this sentence?
No. 얼마나 많이 means "how many". 얼마나 means "how" and 많이 means "many". So it can't be omitted.
얼마나 means "how much" instead of "how", source: Google Translate (Translation verified by Translate Community).
The "how" you are referring to could be 어떻게.
이 중요한 정보를 알려주셔서 감사드립니다! :-)
Leftover word sentence! : Up there wearing good pasta
Only one way to find out...
If anyone's curious, the answer is 206
Could this also be translated as "몸에 뼈 몇 개가 있어요?"
I'm getting fed up with the Korean course. 그/이 isn't in the sentence, therefore a tense (a/the) isn't defined. Source: native tongue.
But the Korean language doesn't always use those "particles" often anyway, so it's not all Duolingo's fault. You should be able to guess them from context