They are the same exact question.

They’re both good, and have the same meaning. The first one might be used if a number of people are expected, and not all have yet arrived. In the second, the word ‘here’ can be omitted : ‘How many of us are there ? ’ and the meaning is the same.

both. but i feel they mean slightly different things: 1 - am expecting 20 people to arrive here. and now i want to know how many of the 20 have arrived and how many are we waiting for 2 - i have no expectation as to the number of people, i am just curious as to how many people are in the room but that's just how i use these constructions