Whenever you cast transplant with the purgatory node it will destroy all skeletons you currently have except for the max amount of “Bone Minions” you have minus 1. So if you have 2 points in Ivory court and 4 skeletons summoned, when you cast Transplant you will spawn one bone minion from Simulacrum and destroy 2 skeletons.
You cannot gain more than one “Bone Minion” from casting transplant if you already have more skeletons summoned than the max amount from Ivory Court. You can spawn up to the max amount of “Bone Minions” from transplant if you cast it with 0 skeletons summoned, then you can summon up to your maximum amount of skeletons.
If you cast transplant again after summoning all skeletons and having maximum amount of “Bone Minions” from transplant spawned, it will again destroy X - 1 skeletons where X is your max amount of “Bone Minions” BUT also destroy all of your currently summoned “Bone Minions” from transplant and then spawn 1 “Bone Minion.”