Well, if you had 2 skill slots you would have far more combinations, though. In terms of sheer number of combos, having half as many slots as skills would maximize the number.
But that’s not really how it would work, because there isn’t enough synergy to support 10 or 11 skills for most builds, so rather than having to choose between, say, bone prison, bone storm, CE, and corpse tendrils, you’d be able to just use all of them and blood mist and both summons. It’s true that might spread your points thin, but with just a single point in each of those you get a lot of their power and you can even get that from items.