I think the maximum number of people per ball is 3, but I could be wrong. We just split into pairs as there were 6 of us.
You can also do a dry zorb and you can choose to be harnessed into the ball or not. If you are harnessed, apparently it's a lot more bumpy. When you're not harnessed, you tend to slide around the ball and stay at the bottom, but when you're harnessed in, you have to roll which ever way the ball goes and you feel every bump - that's for next time
I didn't get any motion sickness but then I have strong stomach - you could always try the under 6 zorbing, where you just roll around on flat ground, you know, break you in gently!