[Lonman]

I was gathering he was asking how many singers would you have from a starting singer to an ending singer before going back to the starting singer (a 'round' or rotation), not how many singers can you get in a night. An average 'round' for me would be 15-20 on weeknights from a begining point to an ending point before starting back to the first singer. On weekends it's about 30-40.
If you are going by how many individual singers do you an average a night, that would be sometimes hard to count it may be the same 15-20 singers all night or with singers coming in & leaving, that number could go higher for actual individual singers, it would be hard to track.
You can only get so many singers total per any night which for a 4 hour period would be about 50-70 singers 'total' depending on the song lengths - so if that is the question, then everyone will pretty much have the same answer based on a 4 hour night.