We have API connection with external system. They cannot use same session and hence will be requesting new session (previous session will still be active) each time they connect with Salesforce.

Can someone please explain:
- What happens to previous sessions?
- Do they expire automatically after 120 minutes?
- At a given time, how many active session there can be?
- What are the implications of multiple open sessions at given point in time? 

regards
Mitesh