After purchasing a license key, there is no limit to how many products can be generated in a feed; any limitations would be server-dependent (php timeouts or limited bandwidth).
If you are generating a feed with a number of products, and don’t see a feed afterwards, look under Manage Feeds. It may just be that your browser is blocking the pop-ups!
In certain cases, if your product attribute is set to in-stock and the quantity of stock is not defined, you may generate feed files without any product information. If so, you can use the advanced commands to set the quantity for products and regenerate the feed files.