As many as you want. There's no way for a server to know when a client machine has a file open, so you have to have your users comminicate if they are going to try to edit the same files.
There are a couple of ways to do "file locking" to prevent two users editing files - I've used Arsenale's Lockpoint in the past (there are other add-ons I think, but I'm not familiar with them)
+1 on Arsenale Lockpoint for file locking. Lockpoint also has a useful feature where it displays WHO has the file open for editing. Other users can then either use Lockpoint's feature to be notified via email when the file is unlocked (i.e. available for editing) or can employ direct means to contact that user if they need to get edit access to the file asap.
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