Vishio enters causing 2 damage, naban sees vishio trigger and causes another 2 damage, panharmonicon sees naban trigger and does another 2 damage(this causes another naban trigger) and then panharmonicon sees vishio trigger and causing a final 2 damage. Grand total of 8 damage.
So yeah 4 triggers tops. I am scratching my head to my initial post to 5 triggers based somehow I got Naban to go off twice and then twice.
Yeah, I had to diagram it out to be sure it wasn't 3 or 5 triggers.
Now, since this isn't meant to be competitive I had considered
helm of the host to make nonlegendary nabans, but there's no way I stay
alive long enough to equip for the resulting glorious cascade of triggers. Maybe
Mirage Mirror...
I'm also thinking about what could happen with
metallic mimic, naban, and
panharmonicon on the board in an attack scenario (offense or defense) with
merfolk trickster in hand and mana open. Trickster is optimally played to tap down 4 blockers and get 4 +1/1 counters for a good blocker for any potential crackback; on defense it can blank keyword abilities, tap down open mana dorks or a defensive creature, or
vigilance creatures but maybe they are already declared as attackers, unless you
flash it in at or before the moment your opponent moves to combat - still gives you a 6/6 surprise blocker either way.