Comment by CookiesOnMyDesk

Something like this, depending on how you want your input and output to be supplied.

    public async IAsyncEnumerable<HttpResponseMessage> SendTwelveRequestsAtATimeAsync(IAsyncEnumerable<HttpRequestMessage> requests)
    {
        HttpClient client = new();
        List<HttpRequestMessage> requestsBatch = [];

        await foreach (var req in requests)
        {
            if (requestsBatch.Count < 12)
            {
                requestsBatch.Add(req);
            }
            else
            {
                foreach (var res in await Task.WhenAll(requestsBatch.Select(client.SendAsync)))
                {
                    yield return res;
                }

                requestsBatch = [];
            }
        }
    }