I think it would depend on the restaurant and just your personal preference. For example, a more high-end/expensive restaurant would probably ask something similar to what you said, "How many are in your party". In a more casual setting, for example, if you're eating outside or somewhere that isn't as expensive, someone would probably just ask "How many chairs do you need?" or "How many". Honestly, there's not really a perfect answer, as long as you're not rude when asking however you want to ask isn't a big deal.