Vantage WebUI server runs in USS under a specific user identity. This user has a limit of the number of files, directories, sockets and pipelines that can be used at the same time - MAXFILEPROC.
Everything running under the WebUI server identity must respect this limit.
This parameter can be set for the user in RACF, TSS, ACF2.
Value 64.000 is generally recommended by IBM - as you can check in the documentation.
However you can set it differently in your environment, based on your needs. For example if the identity that is running the WebUI server is responsible also for running other tasks, that have higher requirements, you can increase it.
Using your Mainframe Access Control product (RACF, TSS, ACF2), you can check the current setting of your MAXFILEPROC.