The enemy amp_id's start at AMP_FIRST_ENEMY and then go upwards from there, until they end at AMP_FIRST_ENEMY+PARTYSIZE_MAX_ENEMY.
So the first monster that was sent out is in global.active_monster_party[AMP_FIRST_ENEMY], the second is in global.active_monster_party[AMP_FIRST_ENEMY+c], and so on.
(IDs that don't have a monster in them has an amp_MONID of NONE, otherwise amp_MONID contains the monster ID and amp_LEVEL the level)
(PARTYSIZE_MAX_ENEMY is 12 by default but if you use the constant to figure out how long arrays you need it should work out regardless)