I've no idea how the Tanker "Higher Target caps" works, if its a buff or what but CoD lists it as 5.
Kind of.
The initial Burn hits 5 for a chunk of damage (and procs) and drops a Burn patch. Each of these can hit 5 people with their patch-style DoT (and more proc chances I guess).
https://cod.uberguy.net./html/power.html?power=tanker_defense.fiery_aura.burn&at=tanker
I think old school Burn didn't have those 2 sets of "cap of 5".
(For what its worth I think Burn is also one power unaffected by the "larger AoE" buff Tanks have now, like I said no idea if / how it interacts with higher target caps)