I'm actually basing this on several extensive discussions here, which resulted in the conclusion that the passage you quoted was misleading. Particularly
here, where KO says so explicitly.
There have been several reports and a couple of incidents I've seen since that make me think those conclusions may not be true. If so, then the only limit on gem use is based on your level. (And I'm only 95% sure on that.)
My suspicion is that casters will use gems to try to keep their fatigue below 100. I had one Crone recently who I had scripted for Arrow Fend, Strength of Gaia, Mass Regen, with just enough gems for the gem costs. (I was using Bards with Soothing Song to drop her fatigue.) So in the second round her fatigue was in the 90s and she used at least one gem casting Strength, keeping her from casting the Regen. In an earlier battle I'd had another Crone start with Strength of Gaia and Mass Regen, and she did them fine, wasting no gems on the booster.
I can't really imagine a D9 caster using multiple gems on Shadow Blast though, unless he'd cast something big first and was already heavily fatigued.
I'll use the BF gem spells, but I won't try to spam things like Shadow Blast. Maybe one to start off. And I'll try to have a caster for each spell to avoid wasting extra gems.
If I had to, I'd assume they were going to use the max on the first spell and give gems to cover that.