Can you give us a link with examples?
A lot depends on how useful, i.e. generic your images are. And of course if you meet professional stock standards in image quality, which are very different from what is requested in the print market or normal art market.
Overall my suggestion would be to choose a mix of 1200 files and upload them to a mix of non exclusive sites over one year. Use the sales you get as a learning experience into what the customers actually like to buy from you.
Once you have identified the customer group that works best for you start producing content specifically for that buyer group and only then would it make sense to consider opening your own webshop or maybe offering exclusive content to more high end places.
Many people come into the industry with : here is my library with 20 000 files...where should I upload them to get the best money?
But the reality is : which type of customer group do I want to shoot for?
It is very, very rare that people have a large library of content on their drives that are actually in demand in the stock market.
And an intelligent use of legacy content is often to mix into a modern, highly targeted new stock production.
Example: you have a lot of excellent travel shots from a location that you love to visit. But you have absolutely no model released content from that area. Letś say, it is a place in the mountains where you love to go hiking.
You could upload your landscape, flower, animal shots as is, that is possible.
But a much more useful thing for the customer would be is if you do highly usable model released, new shootings in that location (and do some research into current styling, colours, image set up, what kind of people are most in demand - families, seniors, women empowerment...) and then mix your legacy files with the new material and processing all of it with the same color scheme, or lens flares, or matte filter effect or more pastel colours etc...
Then you have a really, really valuable group of images and your legacy content can give more depth to the theme.
It is up to you.
You can of course just upload everything as it is, but that would be my suggestion.
First year - pick four agencies and upload a mix of 1200-2000 files, learn about good keywording and description, optimize your post processing workflow.
Second year - look at sales and try to identify customer groups. Read about upcoming events of that industry (sell a lot of food and wine shots?...learn what are the newest industry trends, color and decoration tips, what kind of people for what kind of food, new trendy recipes...)
Start to do highly specific shootings for that customer group. Process legacy content with the best styling and visual trends and mix it into the new shootings.
Third year - Optimize your investment of time and money into new shootings and try to get a good feeling for how long it takes to recoup your investments via sales.
Year four - is it worth hiring assistants to optimize production...etc...
Stock can be a fantastic additional income, but it is all about Useful and what the customers really need.