I don’t know of such a plugin, but it can be custom coded. The topic count by user is not anything that is readily available and determining it will be very resource intensive. An array of the results should be stored somewhere ( a transient for example ) so the results can be quickly output. Only run the calculation on occasion. Every hour perhaps. At most whenever a new topic is posted. A new topic action could simply update the array without running a full calculation. A full calculation could then be run less often, daily or something, to ensure the stored array remains accurate.
A full calculation involves stepping through every single topic and keeping a running count of topics by each author.
Be advised the save new topic action could fire more than once for a single save. The count update routine must account for this possibility. This is why a full validation calculation is advised on occasion.
