Keep in mind though that once that empty space on the planet is occupied by a unit from either side, it is then acquired by the respective player during the Regroup phase.
But yeah, build limit is per order, not per turn.
But only in this case where both sides have a base. You cannot gain a resource card for a planet you do not have a base on. You can lose them, but whomever took it from you doesn't get it without a base.