We recommend having one panel per target group so that the panelists feel that they are part of something which is relevant for them. The panelists will join a panel because they want to contribute and affect something specific, and the engagement in a panel will normally grow over time as you ask even better questions, and panelists give even better answers.
If you have temporary target groups, it is good to know that the monthly cost is based on the number of active panels only. If one panel is no longer needed, you can deactivate it. You then still have access to its content and reports, but can not add more panelists or ask new questions. When you have deactivated the panel you can create a new one for a new target group.