Initial work on supporting multiple islands per player

The default allowed number is 5 for now, but will be set to 1 by
default.

Lots more work to do on this!

src/main/java/world/bentobox/bentobox/api/commands/island/IslandCreateCommand.java

@@ -48,6 +48,11 @@ public class IslandCreateCommand extends CompositeCommand {
             if (island.isReserved()) {
                 return true;
             }
+            // Get how many islands this player has
+            int num = this.getIslands().getNumberOfConcurrentIslands(user.getUniqueId(), getWorld());
+            if (num < this.getIWM().getWorldSettings(getWorld()).getConcurrentIslands()) {
+                return true;
+            }
             // You cannot make an island
             user.sendMessage("general.errors.already-have-island");
             return false;

src/main/java/world/bentobox/bentobox/api/configuration/WorldSettings.java

@@ -634,4 +634,13 @@ public interface WorldSettings extends ConfigObject {
     default boolean isCheckForBlocks() {
         return true;
     }
+
+    /**
+     * Get the number of concurrent islands a player can have in the world
+     * @return 1 by default
+     * @since 1.24.2
+     */
+    default int getConcurrentIslands() {
+        return 5;
+    }
 }

src/main/java/world/bentobox/bentobox/managers/IslandsManager.java

@@ -1891,4 +1891,14 @@ public class IslandsManager {
         return goingHome.contains(user.getUniqueId());
     }
 
+    /**
+     * Get the number of concurrent islands for this player
+     * @param uuid UUID of player
+     * @param world world to check
+     * @return number of islands this player owns in this game world
+     */
+    public int getNumberOfConcurrentIslands(UUID uuid, World world) {
+        return islandCache.getAllIslands(world, uuid).size();
+    }
+
 }