Other than letting a specific ip address to access your FTP server via security policy in ASA, you would also need to look into the FTP software itself at the server. By right it should have some sort of console or management capabilities to let only specific number of users FTP in or user time limit.
I think your answer may well be found using class maps and policy maps.
Check ASA config guides, and command references for more info on how to build these.
Hope that helps :)
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