Is there any command etc available to find out how many concurrent sessions are via a Cisco2821 router? And if yes, is there also a option to find out what is/was the maximum during the day?
Netflow will display the number of sessions through a particular interface.
ip flow ingress
To display the sessions in real time use the following:
show ip cache flow
Keep in mind the information is in hex. Using a netflow collector is best if you'd like to keep statistical information vs. looking in real time on the command line. Short of using a netflow collector the 'top talkers' feature in the link belowmay be useful.