Is there a limitation by hardware or software for Pi to be used by a number of different users?
For example when I ssh into Pi from my computer with my user, I can keep working on Pi also in root user without interacting with other by monitor and keyboard. I think by some other modifications port 23(telnet) and also other ports can be used to connect and work on Pi.
Partial solution after testing
According to Joan's answer I've ran a test and by checking htop|top output figured out each ssh instance was occupying ~3KiB of memory by daemon sshd. So it can be concluded as there isn't a software limit but a hardware limit regarding memory. Also if other equivalents to sshd are used it might take more or less memory so one can have more or less simultaneous connections, a different user took ~4KiB memory via sshd so again most probably there is only hardware limitation.