how get all photos from telegram group ?

I am programming on c#. I need to get all photos I need to programmatically download all the photos. I created bot (/setprivacy disable) and added into this group as admin (). this is my code:
TelegramBotClient botClient = new TelegramBotClient("xxxxxxxxxxx");
var offset = 0;
var updates = await botClient.GetUpdatesAsync(offset);

I have problem with count of returned messages. I need all photos for day (or few days). but I am getting wrong number of messages.

please, help me with it problem.