Port 80 or 23
Because the ONVIF protocol uses HTTP, the RTSP port and ONVIF port cannot be the same.
The ports numbers depend on the camera manufacturer, go to the network setting panel of the camera and write down the RTSP port (used for rtsp://… URLs), the HTTP port and eventually ONVIF port if different from HTTP (used for onvif://… URLs.
By default the RTSP port is usually 554, the HTTP/ONVIF port is 80 or 8080, and the HTTPS/ONVIF port is 443.
(Although some brands of cameras use the same HTTP port for administration and ONVIF, other brands may use a different port for ONVIF and HTTP administration).