Currently only 6 mobs of defined type can be in the spawn area (SoulCageSpawner.java#L96).
Having a limit is good to prohibit lags. But tier 3 & 4 can't use their spawn limit anymore.
Only 6 mobs can spawn at a time, even when using tier 4, which should spawn up to 9 mobs.

I think maybe you should change the line to check l >= tier.spawnCount()