There will be more than 3000 users will upload word document to different folders on my webspace at SAME TIME. How should handle it? I did file uploads but not for more than 4 or 5 user uploded at a time.
If you are looking at 3000 users on a continuing basis you will need a heavy duty hardware and software configuration to handle it. If this is a once per day process (for example), then you probably won't want to invest the time and the cost to build such an environment. There are a couple of links below that address high capacity php-mysql site configurations. You can probably find more through a search. If it isn't a constant requirement, then you probably want to change your approach somehow. Pure FTP may be able to handle a higher volume. Suggest you consult your hosting provider, explain what you want to do and get their input.