The sweet spot right now seems to be 2 Card combinations, which give you a good boost. with 3 and 4 cards, it's more of lesser returns for higher cost, power, noise and temps.
And the more cards you add, the higher your CPU needs to be clocked, to ensure that each card is being fed enough data to keep it active.
Plus, for 3 or more cards, you definitely need an elite cabinet and motherboard that offer 16/16/8/4 x PCIe lanes etc. while 2x 16 or 16/8 or even 16/4 motherboards are pretty common now.
Scaling has improved with each successive generation, but the bigger question is whether the PC has enough games that stretch the capabilities of systems. Back in the day, each release of a game used to hurt your rig, but nowadays, I can run most games on high details at 720p on my laptop with a mobility Radeon 5650 just fine.
So that's that