Which creatures are dealt damage is based on which creatures were declared as attackers during the declare attackers step.

Lightmine Field’s ability triggers when any creatures attack, including your own.

For example: If a creature attacks but is removed from combat before Lightmine Field’s ability resolves (because it would be destroyed and regenerates, perhaps), that creature is dealt damage by Lightmine Field but isn’t counted when determining the amount of damage. On the other hand, if a creature is put onto the battlefield attacking (due to Preeminent Captain’s ability, perhaps), it isn’t dealt damage by Lightmine Field, but it is counted when determining the amount of damage.

How much damage each of those creatures is dealt is based how many creatures are attacking at the time the ability resolves.