I guess it depends on the airline and the sector length/time it takes to fly plus union guidelines ,Also country's aviation authority regs>>
I flew KLM Amst-ATL, and there were 3 deck crew, in AA I always see three deck crew east or west over Atlantic whenever I travel. But I flew Gulf Air from Paris to Dubai one time and there were 2 deck crew. Maybe a Long haul pilot can chip in more.