It's difficult to know, because sometime the course have 100 students, and each day different students make questions to each TA. My guess is 10%-50%, but it depends a lot on the major the students are.
One of the useful trick is to write an optional homework exercise in the blackboard. I get a lot more of answers than just selecting an exercise from the official list. Sometimes it's just an exercise from the list with different number and sometime it has an intermediate step to make it easier. But writing it in the blackboard increase the chance the students will write it down.
Another trick is to write an old midterm and then go to each desk to talk to the students about what they are doing. We usually write the solution at the end of the class, and have some discussion about it, but the useful part is the discussion with each student (or small group of 2-3 students).
