Why do you want to know?
Are you including franchises, or do you mean single companies?
Do you mean nationwide, or within a specific geographic area?
If it's within a narrow area (a city, for instance), do you mean the entire company (whether a franchise or a single company), or do you mean a specific office?
Are you referring to the number of listings or the number of actual settled transactions?
Are you referring to single-family homes, or condos?
Though I'm guessing you're including all price ranges, be aware that the answer might well vary depending on whether the property is low-end or high-end.
If you can clarify your question, I'm sure you'll get some useful responses.