I'm not sure how many there are total. Maybe 6? Maybe not quite that many.
We always just tip based on what we're asking them to do. We'll have them bring a bunch of glasses and ice and even bottles of voda/rum/tequila at times, and we'll tip $20. For most other things they bring or help with is $5 or $10, depending on effort needed.
A couple years ago my wife was REALLY hung over on our last full day there, and I didn't want to waste the whole last day... So I asked the butler to track down a doctor that could give my wife fluids via IV. Believe it or not, he pulled it off! We went to town to the doc office and for $100 my wife got 2 or 3 bags of fluids and electrolytes. She felt great even just by the time we left the office. The butler received a crisp $100 when we got back!!
That was Gerry, by the way. Top notch dude.