So I know that Appian Community Edition users have a limit of 2 Portals, but I haven't found any documentation on limits for other editions of Appian.
Is there a limit to the number of Portals that can be published through the Portal Publishing Manager for a given environment?
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Contractually, there are not stated limits, similar to how there are no contractual limits on the number of Site objects you can have.
There is a back-end limit that will stop you from publishing more Portals once you reach 200. If you somehow reach this limit on a customer or partner site, and have a business case for needing more, please open a support case. While we can raise this limit, it is in place to protect the amount of infrastructure we're managing on your behalf, so if your scenario is more like just having a large number of test Portals on a dev site, you'll likely be asked to simply delete some of the unused ones.