And how do I find out what the other user name is?
I tried cat /etc/passwd
but this gives me a long list of stuff which is basically unintelligible to me.
Top says how many users are logged in. Most probably it is only you, but having two shells open.
If you want to know how many users can login to, just list the /home directory. The system users you should figure out from /etc/passwd, but they are not shown in top.