This is highly dependant on the separation and by that the actual number of different simultaneous skull match “packs” it results in.
Every extra skull match “pack” should add the troops attack to the damage, additional skulls in that same match just add +1/+2 damage.
Someone else can probably explain it better, but the basic idea is, many simultaneous but separate skull matches result in much more damage than a huge cluster of the same overall number of skulls that is counted as one match.
So if say the attack is 20 a 3x3 block of skulls will make 39 damage while 3 separate 3x1 block will do 60? Or a linear 6-match makes 26 vs 40? Does that make sense?
Definitely a single skull match makes at most Att+2 damage, even if it’s a full line/column. The important question is how is a block of skulls split into matches. I don’t know exact algorithms here (most likely only devs know… some of them I mean ;)), but e.g. someone on the forum mentioned that a full board of skulls (from BD or magic-augmented Sheggra) deals only that, Att+2. On the other hand I’m pretty sure that a simple 5-match in T, L or + shape sometimes count as 2 separate matches. Usually when this happened there were more skulls on the board though, so I may be mistaken, maybe there were 6 or 7 skulls in such shape.
TL;DR: sometimes I’m surprised how much damage was dealt, sometimes… well, Att+2 from full board just isn’t impressive, is it?