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Unfortunately, I haven't found an easy way to do this through the C++ or Python API, but I've done it by scraping the output of rostopic list <topic> --verbose.

That will show something like:

Published topics:
 * <topic> [std_msgs/String] 1 publisher

Subscribed topics:
 * <topic> [std_msgs/String] 1 subscriber

The number before publisher/subscriber is the number of nodes publishing to or subscribing to that topic respectively.

I'm not sure why you're implying you can't use rostopic list from your node. Nodes can call command line tools. Unfortunately, I haven't found there doesn't seem to be a an easy easier way to do find this info through the C++ or Python API, but I've done it by scraping API. I just scrape the output of rostopic list <topic> --verbose. in my custom nodes.

That It will show output something like:

Published topics:
 * <topic> [std_msgs/String] 1 publisher

Subscribed topics:
 * <topic> [std_msgs/String] 1 subscriber

The number before publisher/subscriber is the number of nodes publishing to or subscribing to that topic respectively.respectively. You can easily use regexp or simple string parsing to extract that number.