var cookieContainer = new CookieContainer();
for (int i = 0; i < 10000; i++)
{
    cookieContainer.Add(new Cookie("name1", "value1", "/", $"test{i}.com"));
}

var cookieContainer = new CookieContainer();
for (int i = 0; i < 10000; i++)
{
    cookieContainer.Add(new Cookie("name1", "value1", $"/{i}", "test.com"));
}