A script allowing us to limit the number of tiles that will appear in the dungeon to only one (per TileSet listed in the component's properties)

TileSetLimitRule.cs

using DunGen;
using System.Collections.Generic;
using UnityEngine;


/// <summary>
/// Only allows one tile to be placed from each of a group of specific TileSets
/// </summary>
public class TileSetLimitRule : MonoBehaviour
{
	public RuntimeDungeon RuntimeDungeon;
	public TileSet[] TileSets;

	private TileConnectionRule rule;
	private List<TileSet> usedTileSets = new List<TileSet>();


	private void OnEnable()
	{
		// Create & apply our tile connection rule
		rule = new TileConnectionRule(CanTilesConnect);
		DoorwayPairFinder.CustomConnectionRules.Add(rule);

		RuntimeDungeon.Generator.OnGenerationStatusChanged += OnDungeonGeneratorstatusChanged;
	}

	private void OnDisable()
	{
		// Unregister our tile connection rule
		DoorwayPairFinder.CustomConnectionRules.Remove(rule);
		rule = null;

		RuntimeDungeon.Generator.OnGenerationStatusChanged -= OnDungeonGeneratorstatusChanged;
	}

	private void OnDungeonGeneratorstatusChanged(DungeonGenerator generator, GenerationStatus status)
	{
		// Reset out list of used TileSets before the generator starts placing tiles
		if (status == GenerationStatus.PreProcessing)
			usedTileSets.Clear();
	}

	private TileConnectionRule.ConnectionResult CanTilesConnect(Tile tileA, Tile tileB, Doorway doorwayA, Doorway doorwayB)
	{
		// Find which limited TileSet the prospective tile belongs to (if any)
		var tileSet = GetTileSetContainingTile(tileB);

		// If the tile belongs to a TileSet that we want to limit..
		if (tileSet != null)
		{
			// ..and we've already used a tile from that set, disallow the new tile
			if (usedTileSets.Contains(tileSet))
				return TileConnectionRule.ConnectionResult.Deny;
			// ..otherwise, add it to the list so we can avoid placing any more tiles from that set
			else
				usedTileSets.Add(tileSet);
		}

		return TileConnectionRule.ConnectionResult.Passthrough;
	}

	private TileSet GetTileSetContainingTile(Tile tile)
	{
		foreach (var tileSet in TileSets)
		{
			foreach (var tileWeight in tileSet.TileWeights.Weights)
			{
				if (tileWeight.Value == tile.gameObject)
					return tileSet;
			}
		}

		return null;
	}
}