How Many Mechanical Dive Watches Have Depth Gauges?

Oris Aquis Depth Gauge watch

Just a handful. In addition to the aforementioned Oris Aquis Depth Gauge, other mechanical watches with depth gauges include the Blancpain X Fathoms, the IWC Aquatimer Deep Three, the Jaeger-LeCoultre Master Compressor Diving Pro Geographic and the Panerai Luminor 1950 Pangaea Depth Gauge (which  in spite of its mechanical movement has an electronic depth gauge).