これまで最高何人の前で演奏したことがありますか？って英語でなんて言うの？

In this instance you can keep it simple and ask "what was your largest audience?". People will generally respond with a numerical estimate to a question like this. You can also expand the sentence and ask what the was the size of the audience they played or performed to. These also make it clear that you are interested in knowing the size of the audience and should get a suitable response to this.

Crowd is used for a casual expression for audience. "there was such a big crowd last night" "this is the biggest crowd we've ever had!" You can swap biggest for any of other similar word - largest, greatest, etc.

"What was the largest crowd you ever played to?"Here you are asking how big was largest the crowd (group of people) you played in front of (played to). "How many person was your largest crowd?" Is asking how many people made up the audience that you played to. You can ask either of these to know the largest audience size.

How big was the gig? - Possibly musician language. It asks the person if it was either their debut or if it was a big ordeal for them. Also it can be interpreted as asking "how many heads attended?" "How big was the gig?" - "Well seeings it was my debut, there was a good turnout, thankfully!" How many people were there? - Pretty standard. Which can be for a a conference, meeting, vacation etc. What's the biggest number of people you've performed for? - Direct & asking (approximately) how many heads attended.

How many people have you played to? What is the biggest crowd you have played to?

These days musicians have both live and internet audiences, so it may be necessary to distinguish between these two groups.

When you perform by singing or playing an instrument the people you play/act to are called your 'audience' although you can also call them the people you 'play in front of' as they are in front of you when you play