Does only the first non-front unit in a skirmish add its support attack, or do all of them? If I've got, say, one scourge (9 attack) and two devourers (+2 support) going up against a single carrier, is the total attack 11 or 13?
All of them. The rules appear to be quite clear on this matter (page 27).
Each supporting unit is then, one at a time, assigned to any of the skirmishes. Every supporting unit can be assigned to the same skirmish, or spread out among the skirmishes in whatever fashion the player chooses. A supporting unit adds its support value to the attack strength of the friendly front-line unit in its skirmish.
Important: A player must assign all of his supporting units to skirmishes. That is, every unit in a battle must participate in a skirmish, either as a front-line unit or as a supporting unit.
Supporting Units: Each supporting unit in the skirmish adds its support strength (or other effects) to the player’s attack value. A unit’s support strength is listed on its owner’s Faction Sheet.
There is only one exception that I could find, but it isn't related to the number of supporting units. It is related to a support unit being unable to target the opposing player's front-line unit (page 34).
As explained under “Compare Attack and Health Values” on page 31, a supporting unit adds its support value to its controller’s final attack value for the skirmish in which it is participating. However, this is only the case if the supporting unit is capable of targeting the opposing player’s front-line unit in that skirmish. If the supporting unit is not capable of targeting the opponent’s front-line unit, the supporting unit does not add its support value to its owner’s final attack value.