It depends entirely upon what you mean by “extensions” and “user.”
If you are using chan_sip extensions, you can only have one device registered at a time to each extension. So, Extension 101 can only have ONE phone connected to it. Extension 102 can likewise only have ONE phone connected to it.
The FreePBX devs developed a number of work-arounds to allow you have to multiple phones that appear to have the same extension, but none is perfect. First, there was “device and user mode,” which was the most promising, but which the devs continue to state is “not supported” or something like that. I’m not going to go into any detail beyond that.
The second work around is to set up each phone with its own extension number, but then have a ring group that rings all the phones. You’d also set the internal caller ID field for each phone to be the ring group. You’d also set up only one of phones with voicemail, and then use the mailbox field to cross-reference the other phones to that extension’s voicemail so that everyone would show the contents of that voicemail box. This is the easiest method, but it won’t support force all internal auto answer if you use that feature.
Pjsip extensions are supposed to support multiple devices registered to the same extension number at the same time, but I’ve been told that the SIP headers will get split if you have too many and they go out over the public internet. I don’t know much about that otherwise.
If by extensions and users, you are referring to how many different extension numbers can be registered to a single phone, then the limit is based upon the capabilities of the phone.