When the open_files_limit is not high enough for the number of tables present mariabackup will at some point start to spew out error messages like:

2023-02-08 11:12:28 0 [Warning] InnoDB: Cannot open './test/x55800.ibd'.

2023-02-08 11:12:28 0 [ERROR] InnoDB: Operating system error number 24 in a file operation.

2023-02-08 11:12:28 0 [ERROR] InnoDB: Error number 24 means 'Too many open files'

2023-02-08 11:12:28 0 [Note] InnoDB: Some operating system error numbers are described at https://mariadb.com/kb/en/library/operating-system-error-codes/

It will then continue to throw the same lines for any further table it tries to open, which can be a lot.

As at the point where this is being hit for the first table it is already clear that a consistent / complete backup is no longer possible it could, and IMHO should, bail out right away instead of further spamming its output with repeated messages about the same problem.