Good point - I'm not sure, actually.
If I'm not fighting for the scrying pools, 4 is probably enough, although with +buy and a potion doing nothing for me, I'm likely to pick up more even after they stop being my main focus (without +buy, the SP is much weaker and I probably shouldn't be going for them in the first place.) If I'm fighting, I'd rather have more than fewer, but I'm not too sad with a 4-6 split, and in this game, I wasn't sad at all thanks to scheme.