I’m experimenting with a Shapeshifter Evoker setup that should hypothetically be able allow my team to equip 23-26 spell gems:
2 from Artifact’s Trick Slot
5 from Expanded Mind
13 from Master of Shapeshifters (6 base, 3 from Forest Pact anointment, 2 from Odd One, 2 from Shackler’s Mastery)
…and Spell Tap for an additional +3 on one specific creature
However, it appears that the number of spell gem slots caps at 18, even if you should have more. This doesn’t appear to be because of any specific effect in the lineup failing; I tried switching various bonuses on and off in different combinations, but they always stopped at 18.