How many files does InnoDB have open?

With innodb_file_per_table = 1 and a finite innodb_open_files, table growth eventually presses file-descriptor limits (open_files_limit included). Older MySQL builds did not expose the open .ibd count via SHOW GLOBAL STATUS or SHOW ENGINE INNODB STATUS.

Practical check

# all ibd opens on the host (coarse)
sudo lsof | grep -c '\.ibd$'

# scoped to the mysqld PID
lsof -p "$(pidof mysqld)" | grep -c '\.ibd$'

Raise innodb_open_files (and OS limits) before you hit EMFILE under DDL or traffic.

MariaDB/XtraDB later added Innodb_num_open_files — prefer an in-server status variable when your version has it; keep lsof for hosts that do not.