Theres 2 main benefits a 4 valve has over a 2 valve.
1) Flow area - for a given valve lift, the 2 smaller valves have more area available, which allows for opening/closing points closer to optimum for the engine use case.
2)Weight - it requires less spring pressure to control each smaller valve, and so the engine redline can now be higher than a 2v (typically)
Theres other benefits as well, and some downsides (port induced swirl vs tumble) but even going to a 5 valve (3 intake) had some weird port flow behaviour that made its mechanical complexity too much of a cost to continue (yamaha has abandoned the 5v afaik)
With 4 valves, the mechanical limits can already be shifted down to the bottom end limits - rod bore integrity, piston, oil pump rpm. Adding more valves wont change that.
Then theres the fact that engines arent just sheets with their specs on them. You've got its external size vs internal size to consider, as well as its center of mass, such as gm still using a pushrod v8 in its highest performance car (they blind tested the c5 with both a dohc and the in block cam, the lower cg of the pushrod engine had more favourable results).
Now, honda did do a v4 with 8 valves per cylinder:
https://en.wikipedia.org/wiki/Honda_NR
Theres a reason that no one has tried to make that a production engine.