Equally spawning assets around terrain

Question

I have a terrain I have generated and I am looking to equally distribute assets around this terrain so there is not more of some assets and not many of another. I am not sure how I could approach equally distributing 6 unique assets equally around the terrain.

I am not sure if calculating the percentage of an asset appearance by doing Unique asset/Total Assets is the right approach.

To spawn objects (spawning collectable coin in this case) I am currently generating a random position between 0 and the size of the terrain and randomly giving it a chance to spawn. The terrain has been generated with a Grid which allows me to place the objects at a valid node position so that a path finder can access it.

I am looking to have a set of 6 unique assets and distribute them equally around the terrain.

Quite new to C# and Unity and was wondering how I could approach this. Would appreciate any insight.

TerrainGenerator.cs

Grid.cs

 if(SpawnCoins)
    {
        int range = 3;
        var x = Random.Range(0, GridData.Width);
        var y = Random.Range(0, GridData.Height);

        var NodePosition = GridData.grid[x,y];
        if(NodePosition.terrainType == TerrainType.Grass || NodePosition.terrainType == TerrainType.Sand)
        {
            if(Random.Range(1, range) == 1)
            {
                if(!GridData.IsCellOccupied(NodePosition))
                {
                    if(SpawnedCoins < SpawnCoinsAmount)
                    {
                        Vector3 spawnPosition = new Vector3(NodePosition.Position.x, NodePosition.Position.y, NodePosition.Position.z);
                        var coinAsset = Instantiate(Coin, spawnPosition, Quaternion.identity);
                        coinAsset.transform.SetParent(this.transform);
                        SpawnedCoins++;
                    } else {
                        SpawnCoins = false;
                        return;
                    }
                }
                range += 10;
            }
        }
    }