We are working on a project that may require up to 10000 databases. We are looking at MongoDB as an option but not sure if we can create that much databases.
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There is no hardcoded limit on the number of databases one can create in MongoDB. However, there are other limitations that one needs to be mindful of while creating a database. For example, there is a database limit of 100 if you are planning on using Atlas (M0, M2/M5 tier clusters). You also need to be mindful of your resources like RAM, Memory utilizations, etc.
You can read more on the Documented limits & thresholds in the MongoDB Limits and Thresholds page.
There is also a similar conversation that you might find useful: