Completing open-ended questions is very time-consuming, especially at the end of a survey. Unless the questions are on topics the audience has a burning need to communicate to you, you should not expect a high response rate.
How Many Participants to Choose
No matter how many people you involve in focus groups, your results will not be quantitative. The goal of focus groups is to get qualitative information on different issues.
Which Participants to Select
What you need is to listen to a cross-section of your audience to hear what you haven’t heard before from the people who don’t usually come across your path.