PC is clearly huge for the series. But it's not the majority.
You're looking at around ~40%, give or take, of series sales coming from the PC versions of the games.
But as noted earlier, the Souls series has benefited from releasing on PC much more than other games.
It's 4m+ for PC out of 9m+ (because there's 8 months of consoles sales missing from the total).
For some reason i was sure it was 5m, my bad then