Theory of Games
Storied Gamist
Ok. SO a group of 30 Kobolds = 3000 XP, using the EM. Considering the DPR of five 6th-level PCs who probably have magic items (and at least one arcane caster), 30 kobolds is a joke. By the same tables, 6 Kobolds would be a HARD encounter for four 1st-level PCs.So... there's a "multiple monster XP modifier." In scenes with lots of monsters you don't use RAW XP for encounter building maths in 5e – it gets adjusted by this "multiple monster XP modifier" due to just how big of a factor having an action economy advantage is.
I'll attach a snapshot of what I'm referring to from DMG page 82.
This is not perfect – there's also newer encounter building stuff in Xanathar's – but this multiplier IS something that my experience tells me is important to factor. Again, I'll reiterate that the WotC maths for Encounter Multipliers in the DMG is... it's not precise... do TWO monsters really need a multiplier? No that's dumb... but it's the general IDEA around comparing action economies that's important to keep in mind.
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For example, 15 bugbears will sum up to 3,000 XP, but then the x4 multiplier makes the threat they present to a typically sized party of PCs more like 12,000 XP. The DMG makes this sound precise. It's not. But the point is they're much more threatening than the 3,000 XP you assumed because they get so many more actions than the party.
Going back to my method – total adventuring day budget for this part of Five 6th level PCs is 20,000 (Adjusted) XP.
My experience is that throwing ~50% of that total (Adjusted) XP at a 6th level party in one encounter is going to be pushing them to their limits. So the 15 bugbears would – obviously depending on circumstances (e.g. 150 feet away in open terrain with two PCs casting fireballs? the party surprising the bugbears? etc) – be much more likely to play out like a brutal scene where they are fighting for their lives and having to pull out all the stops.

The whole thing is silly because the tables assume the capabilities of the PCs. And knowing how powerful arcane magic is at all levels, I'd ignore the multiplier and give the monsters a chance. One thing old-school players (BECMI/AD&D) learned was some encounters you fight, some you negotiate, and some you run from.
But, that's just me.