Adding nodes to ros topic

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Yes, it is possible for many ROS nodes to subscribe to one topic.

I don't believe there is a hard upper limit to the number of subscribers; the limit will mostly come from the performance of the computer. As a rough guess, I think it should be possible to have over 100 subscribers on a single topic; possibly many times that.

I'm not sure any testing has been done to validate the maximum number of subscribers or the performance impact of having very many subscribers. If you want to have lots (>100) of subscribers, you should write a simple test and validate that the publisher and subscriber work well in a scenario that is representative of your use case.