In stock photography (and Alamy) terms I'm still very much a newcomer, with only 3 years portfolio building since I joined Alamy in January 2014. I'm not with any other agency. So understand that anything I say is based on that limited experience.
It's certainly possible to make a small income from Alamy sales. However, it takes time to build up, and it's certainly dependent on image quantity, quality and appeal to likely buyers. I think I've been reasonably successful, with 203 image licenses reported to date, but that's not generating a lot of money. The average net monthly payout for this year - 4 payouts, January to April - is just over $100. Useful - but hardly enough to call it a living. It's also been hard work. Shooting, processing, keywording, and uploading all take time and energy to add the 100 images a month I've averaged.
I am, admittedly, shooting mostly in a specialised area. But it's something I know a lot about so the accurate keywording and captioning that will draw the buyers comes very easily to me. Of course, once images are seen, they have to be good enough to capture the buyers - but that's the case in any area of stock. I appear to be doing well enough to have a rolling year average CTR of 1.04, nearly double the Alamy average, and giving me around 2 - 3 sales per 1000 images per month. Even so, $500 - $700 a month net earnings looks way in the future. I'm going to need a lot more images of similar or better quality than I have now. As one of the other contributors has remarked in another thread, "it's a marathon, not a sprint". Don't expect too much, don't expect it for some time, and don't rely on it. It's easy to have bad months where you earn virtually nothing.