Basically the option that control the number of calls one extension can accepts at a time is in sip_additional.conf
but you cannot edit this file directly as each time asterisk restart / freepbx restart, this file will be reseted and your changes will be gone.
The work around is to make use of this file call sip_custom_post.conf
and instead of copying the whole extension configuration file here, all you need to do is to add the following lines
**620 is the extension number you want to override the call-limit option. **