Tell me how many heads you have...,

[Silverobama]

Thomas and I are supposed to meet in the mountains, and we’re planning to go to a famous restaurant there. Since the restaurant is usually packed, I need to make a reservation in advance. There will be three people on my side, but I need to know how many people will be coming with him because I’m not sure whether his wife and daughter will join us. So I said to Thomas:

Tell me how many heads do you have so that I could book a compartment room beforehand.

Please help me with a natural version.

 

[emsr2d2]

Thomas and I are supposed to meet in the mountains, and we’re planning to go to a famous restaurant there. Since the restaurant is usually packed, I need to make a reservation in advance. There will be three people on my side, but I need to know how many people will be coming with him because I’m not sure whether his wife and daughter will join us. So I said to Thomas:

Note my changes above. You don't need "a reservation" and "in advance". Reservations are, by definition, in advance. I think we've told you before not to try starting sentences with "so".

Tell me how many heads do you have so that I could can book a compartment room beforehand.

Note my corrections above. As far as I'm aware, the majority of human beings have just one head so this question made me laugh out loud! Although restaurants themselves might refer to diners as "heads", we don't use it in everyday speech.

How many people will be in your group? I need to know how many I'm booking for.

I have no idea what a "compartment room" is. Do you mean a private room? Is it like a booth?

 

[tedmc]

How about;
How many of you are there, so that I can book a private room?

 

[Silverobama]

I have no idea what a "compartment room" is. Do you mean a private room? Is it like a booth?

Yes.

But I don't think it's necessary to use the phrase after reading your suggestion.