public dynamic List()
{
    var categories = db.Categories.OrderByDescending(x => x.CreatedDateTime).Select(x => new
    {
        ID = x.ID,
        Name = x.Name,
        RootCategoryName = x.RootCategory.Name,
        ItemCount = x.Items.Count() // This line
    });

    return Json(categories, JsonRequestBehavior.AllowGet);
}

public class Item
{
    public int ID { get; set; }

    public int CategoryID { get; set; } // Seems like here I've done everything correctly
    public Category Category { get; set; } // Seems like here I've done everything correctly

    [Required]
    public string Name { get; set; }
    public string Info { get; set; }

    public bool? IsAccessory { get; set; }
    public int? RootItemID { get; set; }
    public Item RootItem { get; set; }
}

public class Category
{
    public int ID { get; set; }

    public int? RootCategoryID { get; set; }
    public Category RootCategory { get; set; }

    [Required]
    public string Name { get; set; }
    public string Info { get; set; }

    public IQueryable<Item> Items { get; set; } // Here also it seems that I've done relations correctly

    public DateTime CreatedDateTime { get; set; }
    public DateTime ModifiedDateTime { get; set; }
}