ServerManager.cs

using System.Collections.Generic;
using UnityEngine;

public class ServerManager : MonoBehaviour {
    private Dictionary<int, ClientData> clientList;

    [SerializeField]
    private GameObject clientPrefab;

	// Use this for initialization
	void Start () {
        clientList = new Dictionary<int, ClientData>();
        
        //ClientData型のクラスをリストに格納
        for(int i = 0; i < 3; i++)
        {
            //クライアントデータを格納するプレハブの複製
            Transform clientObject = Instantiate(clientPrefab.transform) as Transform;
            ClientData clientData = clientObject.GetComponent<ClientData>();

            clientData.id = i;
            clientData.itemList = new List<ItemData>()
            {
                new ItemData { id = 0, power = 100 },
                new ItemData { id = 1, power = 110 }
            };

            //クライアントデータをリストに格納
            clientList.Add(i, clientData);
        }


        //格納したデータを表示
        foreach (var data in clientList.Values)
        {
            List<ItemData> items = data.itemList;
            foreach(var item in items)
            {
                Debug.Log(data.id + "  " + item.id + " " + item.power);
            }
        }
	}
}