Tipping is always gladly appreciated, but never required or expected, so there’s no standard amount. If you would like to tip your girl, then give her whatever you feel like she earned.
Multiple girl parties are very common, and the only limit to how many ladies you play with is how much you are willing to spoil us. We all have our go to girls that we are comfortable playing with, so it’s best to find a courtesan that you like the most and let her find you a few friends.