I had a similar issue recently, where out of 4 possible types, only type A could be added once. You can put the following in a ContentSavingNotification handler ( https://docs.umbraco.com/umbraco-cms/reference/notifications/contentservice-notifications ) and tweak to your needs, hopefully will be enough to get you going

 foreach (var content in notification.SavedEntities.Where(c => c.ContentType.Alias == "yourNodeAlias"))
 {
     var itemsJson = content.GetValue("items").ToString();
     var blockValue = Newtonsoft.Json.JsonConvert.DeserializeObject<BlockValue>(itemsJson); //umbraco uses newtonsoft not system.text
     var amountOfYourTypeOfBlock = 0;
     foreach (var item in blockValue.ContentData)
     {
         var contentType = _contentTypeService.Get(item.ContentTypeKey);
         if (contentType != null)
         {
             if (contentType.Alias == "imageLink")
             {
                 amountOfYourTypeOfBlock++;
             }
         }

         if (amountOfYourTypeOfBlock > 1)
         {
             break;
         }
     }

     if (amountOfYourTypeOfBlock > 1)
     {
         notification.CancelOperation(new EventMessage("Error", "Please select 1 type only", EventMessageType.Error));
     }
 }