Back in the day ( I’m being facetious) there were 6 run levels.
0 System Halted
1 Single User Mode
2 Multi User No Networking
3 Multi User No X
4 Not Defined (special use cases)
5 Multi User with X
These days Ubuntu is the distribution of choice. Unlike its predecessors, it does not have as complicated a scheme. In Ubuntu (and Debian as a base) there is no distinction between runlevels 2-5. 1 is still single user mode and 6 is still reboot. On top of this wild and crazy scheme, Ubuntu has gotten rid of inittab. Prior to 9.04 entries were put in /etc/event.d but with 9.10 even that is gone. They call their system and it can now be found in /etc/init . For administrators moving from an older distro, it can take some getting used to.