I would like to detect open files error in advance and for that I need the current number of open files.
Unfortunately I do not have access to the system and would like to do that via cql.
Is the number of sstable_tasks in progress the same as the number of open files or is there another way?
The number of open files isn't stored in Cassandra's virtual tables and there is no way I'm aware of that you can infer it from other metrics stored in the virtual tables.
If you are a DBA for the cluster, you really require access to the servers as a requirement for performing your role. There is no way around it if you want to do it properly. Cheers!