I just used the choice activity to have students preregister for some courses. There were a variety of limits for the different sections, but in some cases the number of students exceeded the set limt. For example, I set a 20 person limit for the Monday 2nd period section, but 29 were able to select that section. This only happened to 3 other sections and they were ones with higher limits. I don't know if this had anything to do with it, but the server was hit pretty hard. When the activity opened there about 300 users trying to make selections and the server really slowed down. A couple of students also got database errors.
Did you really set a 20 person limit? please send me the screen shots so that i can help you
problem will be due to groups. seems it allows limits set for each group.