Reseted my build to test it (replaced it before for Flame Ground, which is really good), and I was wrong.
At level 5 Fire Wall has 3 tiles and the skill description says a total of 7 hits, but actually each tile can hit 4 times before disappearing. (that 7 has no meaning whatsoever, may be another visual bug tho, like the “0 damage”)
Having more than one monster on the same tile will consume it faster (no AoE damage, each monster will count for the maximum number of hits).
If the monster is placed between tiles it will get hit by all close tiles, consuming maximum number of hits from all of them too.
Probably (lazy to test more, reseting builds is boring and I have done too much of it… :c) this maximum number of 4 hits is default, and leveling the skill only gives more tiles (every 2 lvls) and base damage.
The only change FW got after iCBT ended was the reduction of cooldown, to 45 seconds.