Guillaume,
If I understand you correctly you are trying to figure out how many unique groups (and therefore communicators) were globally created as a result of an MPI_Comm_split_type operation. Unfortunately, depending on the type passed to this function (there is a single one defined by MPI, but in Open MPI we support several extensions), the resulting communicator can be created without the need for global knowledge, and thus it is impossible to know how many have communicators have been create in total.
To extend on what JeffH proposed, you can count the resulting number of communicators by doing an MPI_Allgather on the initial communicator on a data where each participant provide their rank in the newly created communicator (then returned by the MPI_Comm_split* operation), and then counting the number of 0.
George.