In a game of D&D 5e last night we had to creep through a room containing ~1000 Duergar.
It should have been easy, as they were already distracted, but our Paladin, who we'll call Clanky McClankface, managed to get the attention of one of them.
Off we run in to a corridor pursued by the Duergar and exit on to a set of stairs. Imagine the stairs in Moria in Lord of The Rings, but each step is ~6 foot, and slippy.
Clanky makes up for his earlier mishap by summoning a Champion to hold the doorway which fits 2 dwarves at a time, while we start to descend. I decide we need a bigger buffer so throw a fireball in to the mass...
In this case the DM decided it was reasonable to assume 50 Duergar met a fiery demise so, woo hoo!, 10,000 xp for me.
It did get us thinking though; fireball is a 20ft radius so ~1256 sq ft. Had I thrown it into the original mob and we assume 5ft of space for each dwarf that would be 250 dwarves and 50,000 xp (!). As it was I went up two levels from 5th to 7th but 50,000 would have put me in touch of 10th level. From one spell.
If GMing would you argue it would be reasonable to cap the xp from such an event?
For clarity; I'm more than happy with how it played out.