Now, I have any 20,000 files in single directory. How many maximum files per directory on CentOS 64bit ext3?
The number of files in a directory is limited primarily by the maximum file size, divided by the size of a directory entry. As the maximum file size is in the terabytes range, there is no practical limit on a directory size. Depending on what ext3 features you use, performance may degenerate as the directory grows larger.
There is a limit of 32000 subdirectories.