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There are different "problems" as you're saying that could tackle your needs.

- Gesture recognition to detect different hand gestures

- Skeleton detection to infer people's positions and whether they have a raised hand

- Object detection (e.g., YOLO) to detect hands

I believe in this case you could go for skeleton detection as you get information from which you could infer raised hands.

right but can skeleton detection do hundreds of people at once?

You can always slice the images into smaller ones, run detection on each tile, and combine results. Supervision has a utility for this - https://supervision.roboflow.com/latest/detection/tools/infe..., but it only works with detections. You can get a much more accurate result this way. Here is some side-by-side comparison: https://github.com/roboflow/supervision/releases/tag/0.14.0.