15 is max, but the game is bugged (go figure) Sometimes it stops at 9 in first step despite still having 1:30 on the clock, Sometimes the "Ritual Circles appear faster" doesn't actually work, sometimes a circle during 2nd phase doesn't pop up. But 15 is the max. 10 in phase one, 2 during first shield phase, 3 in second shield phase.

15 WILL spawn… 10 before boss, initial 2 after boss spawn, then 3 in very quick succession (almost simultaneous).

Didn’t spawn 15 the last time I played Europa. Killed the 10 and killed another 4…5th one didn’t spawn, but I still cleared that part of the quest for that triumph in Europa 🤷‍♂️

I completed the triumph Wednesday night /Thursday morning.. we got 15 to spawn each time, in each location. 🤷‍♂️.

You're lucky mate. The moon haunted lost sector bugged out 2 times for me this morning and only spawned 9 headless instead of 10. In the 2nd time around, I was stood at C plate and when the timer got to 100 no headless spawned and instead the timer just reset to zero and I had to get it to 100 again. I'm picking the headless that was supposed to come out the first time was the one that would've given us our 10 kills.