I've always heard that crowding plants together is unlikely to improve yield. But I haven't compared it myself, so I can't really say.

I wouldn't. Melons/gourds/squashes are all big growing plants with a correspondingly big root system. One will easily fill a 5G pot and adding more will just crowd it. If you want more, plant them separately in their own suitably sized container.

More plants will not result in a higher yield. A bigger container for that one plant might result in a higher yield. I grew bitter melon last year as a favor for a friend. It is a MONSTER. 5 gallons is barely big enough, if that. I would do a 20 gallon container minimum - the fabric ones are quite affordable. Plan to trellis it and then have it exceed the trellis.