I have killed the ender dragon 19 times in my hardcore world but i only have 9 gateways around the main end island. When i killed the dragon for the second time the game "respawned" the portal i got when killing it the first time destroying a bit of the platform i built around it. And then when i continued to kill the dragon, sometimes spawned a new gateway and sometimes not. The last 5-6 times it didn't spawn in a single new gateway.