the 11504 is all proxies scraped from sources.txt. A lot of those could be dead or not anonymous. Once it finds a proxy that is suitable, it regularly checks it. When the time comes to dump to the text file, it takes all proxies that are currently working, and dumps those to the text file, overwriting the old proxies in the text file. So, as it runs for longer and longer, it finds more reliable proxies, so the number dumped to the text file goes up.