Hola Darryl -
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The first thing is to ask yourself is "short-haul" or "long-haul flight"...
My airline acquired 747-400s a few years ago, to replace our 747-200 fleet. The airline management obviously decided... Payroll only 2 rather than 3...
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With our regulations regulations (Argentina uses US/FAA regulations) - the limit of flight time for a crew of 2, is 8 hours scheduled flight time, and 12 hours duty time (which starts 90 minutes before scheduled departure and ends 30 minutes after scheduled arrival). And to the surprise of the management, we are never able to fly a 747-400 to fly with its minimum crew, in view of flight and duty time limitations. The 747-400 is generally a long-haul aircraft... maybe not with the 747-400D in Japan...!
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For flights up to 12 hours, we need to include 1 relief pilot. For flight in excess of 12 hours, we need to include 2 relief pilots. Relief pilots generally sit in the flight deck for departure and arrival to give a hand when needed. So that is the reason you see "many heads" in 747-400s... unless maybe operating a flight between LHR and LGW...
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You might see only 2 heads with a 737 or 757 around in UK, obviously these are short flights. An extra pilot might be a pilot deadheading and being there for "social" reasons, or a check pilot to observe the cockpit crew.
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Myself, when doing personal travel, I visit "the boys" up-front, and stay with them to help and add my two eyes for traffic on departure and arrival... In cruise, I go relax in the cabin and taste the paté de foie gras, the lobster and wine and cognac, and discuss with the cabin crew "how tough" life is with the airlines nowadays... and yes, I might wear jeans and T-shirt, no uniform is required to go to a Brasil beach...
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And, 10 months to go for retirement...
Happy contrails