What?20 encounters
I was gonna say this. If the PCs go four days without a combat encounter and then have one on the fifth day, is that an average of .2 per long rest? Or do the 4 days don’t count?The question probably should ask "In adventuring days involving a meaningful amount of combat, how many...."
Because now you have people answering "we have entire days with no combat" skewing the averages for a different kind of question than the one you intended to ask.
A simple question,
in 10 years of 5E, how many combats did you have per Long rest in 5E?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.