A Foundry world is a self-contained instance of content, usually making up a campaign or one shot. It is created in the setup menu within Foundry VTT, or within the “My Foundry” page on The Forge (for users with access to our The Game Manager feature).
Only users subscribed to The Forge can create their own worlds.
As many as the user wants. Or rather, as many as they can fit within their account. Most worlds do not consume a large amount of data, and The Forge supports exporting world data at any time. Reasonably, a Forge user would have to create dozens of worlds, with a significant amount of game data in order to come close to reaching their data quota- at the lowest possible tier that can be subscribed to.
If you would like to have more than one world active on The Forge at once, we allow users to do so through our Story Teller tier. Please keep in mind that although the world will be active, users cannot access more worlds than the number of Foundry license keys on your account at once. This is due to the licensing terms of Foundry Virtual Tabletop software, and we cannot change these terms.