The rule of thumb is that you need 1 kit for every MIDI cable you intend to replace. If you are connecting a MIDI keyboard wirelessly for use on a stage, or just wanting to remotely drive a player piano or organ, 1 kit is sufficient with regular MIDI cables.
If you have traffic going both directions to the same 2 devices, 1 MIDIjet Pro USB is sufficient (or 2 of the original MIDIjet Pro). If you are connecting to devices with 5-pin MIDI in and out ports, you will need a bi-directional MIDI cable. If you are connecting to a device that has a USB A port (flat and rectangular), you only need a USB A to B cable.
Please see the Applications page for more details.