I'm going to try shooting original content just for stock from here out and consider it another wing of my business ( I shoot advertising and corporate events ). I know financial prospects aren't high but the plan is when I have free days ( which are endless currently )

My two cents:

1) Search for simple concepts like sick, education, moving, business, office, work, energy, shopping, etc, and look for what sells at the micro stock sites -- sales volume is the biggest factor that affects the stock sites ranking algorithm. So pay attention to the first few pages of results.

2) If you have the time to dig a bit deeper, use Google reverse image search (with Chrome, right click on an image, and click "Search Google for image") to find out how the image is used. Once you have an understanding on how an image is used, then you can start from there and develop your style that possibly outdo the original photos.

3) Take photos of objects that you have an unfair advantage on. By that I mean the city you live in, event locations you have access to, etc. While other photographers may have already uploaded photos of your city, you have an unfair advantage on when you take the photo. In fact, you can capture photos with the right time, mood, season, etc.

4) Lastly, you may also want to upload editorial photos? Consider uploading photos of hot companies (Amazon, Microsoft, Apple, etc).

I think $500 a month can be achieved (especially when the economy gets back to normal). If you focus on creating highly sellable images instead of simply by volume, you can get there way faster with a lower number of photos than others have suggested.

Most of the photos hosted by the free stock sites are shot by the cheaper cameras, like the APS-C and/or the older, entry-level level ones:

https://pixabay.com/en/cameras/

I have a co-worker who contributed a small amount (less than 50) of photos on pixabay, and he shoots with an old D5100. He gets a few dollars a month from people who download his photos, he is not for the $, mainly just for the fun & enjoyment. I guess that's why others are contributing to the free stock sites.

Enjoy: https://www.youtube.com/watch?v=mQ595End5Xw