There is a database used as the Model provider to store ThingWorx's metadata and system data, and there's only one for each ThingWorx installation.
And there are databases used as Data Providers for ingesting stuff. You probably meant those.
If so, I don't have a straight answer to the question - not sure if there is a hard limit, and it's definitely possible to create more than a dozen.
Can't think of a reason to use that many, though. Where does your question come from? Why would you want to add more than a few additional databases? Do you have a really-really high load use case?
Just as a side note, I'd suggest to take a look at these chapters in the documentation:
https://support.ptc.com/help/thingworx/platform/r9/en/index.html#page/ThingWorx/Help/ModelandDataBestPractices/persistenceProviderOptions.html#
https://support.ptc.com/help/thingworx/platform/r9/en/index.html#page/ThingWorx/Help/ModelandDataBestPractices/DataManagement.html#
