More than one
As I read down what people suggest, i noticed most said don't do it. Don't run with 20th to begin with and of course that make sense but since you went along anyway (after all its a fun challenge) and was actually having success with it since you and I guess your party liked it, then keep it up.
However I read what has happened with the counters, one took a while then other downed in seconds. both are actually fine. However what I noticed was the party was "together" too strong for the single encounter which made me think : MORE than one - make an encounter where one of the creatures are great at taking down ranged attacks - one that hits hard for the monk and one whom are superior when it comes to divine magic. Even throw in a trap now and then (didnt see any rogue to disarm those)
Basically what I would do was, take the party to some remote place and hammer at them over and over again till they run out of "energy" using small to medium multiple encounter groups. I once many years ago read an adventure for a party of avg level 6 going into a goblin cave. I seem to recall the intro was something about if Goblins are so easy to kill why are there so many of them.... that always stuck to me. Even the greatest of warriors run out of steam and then suddenly the party might be in trouble.
But more important with all this - KEEP running your minicampaign and get valuable experience. Its only getting better