In Linux, there are some cases that application needs to open multiple files at the same time - probably database like MySQL is one of the cases accessing multiple files at the same time, because MySQL create files for table and its index on the file system, so it will cause opening multiple files at the same time.
By the way, there are some way can check and increase the maximum allowed files you can open at the same time on Centos.
Find Linux Open File Limit
The number you will see shous the number of files that a user can have opened per login session. The result might be different depending on your system.
Check Hard Limit in Linux
Check Soft Limits in Linux
Set User Level Open File limits in Linux
You may want to apply limits per user basis like apache, mysqld, and etc. For that purpose, as user root, you will need to edit the following file:
To increase soft and hard limits, please add following lines at the end of the file.
Once you login the machine again, your soft/hard limits will be changed like you set above.