Where to find the various cargo hold dimensions and volumes of transport aircraft, such as the A330-300?
The [relevant] dimensions of the cargo compartments are published by the manufacturers. For all Airbus planes, visit: Airbus / Airport and Maintenance Planning. You can also search for other manufacturers.

For the requested number of containers, here they are for the A330-300/900 (no need for the volume as the containers are standardized):



The documents also include the locations of cargo doors.