Well technically you can get more. You get d3 for having Slann or starseer general, d3 for astrolith, and then you get d3 per Slann or oracle if you choose to not cast. Theoretically you could get as many d3 as you include oracles and Slann, but oracles are I think the worst warscroll in the book, and Slann have diminishing returns due to not being able to cast spells more than once and needing body guards to not get sniped off the board.
And even if you go all in, you could roll all ones on your d3 and summon a mighty unit of 10 skinks for your troubles (womp womp).
So basically, the idea is to not build your army around summoning any powerful units. You should include your core that you are building around, and just use summoning to fill out the chaff and objective grabbing ranks as the game goes on. Seraphon are now much more about command ability upgrades during battle and spell casting.
Personally I really like that summoning was hard nerfed. I find list building and playing the army to be much more interesting now.