sudo useradd username
In order to understand users, first lets have a look at the current users in the system
ls -l /home
In order to see all users, you have to go to the file /etc/passwd. So lets do that:
cat /etc/passwd
If you want to count the users, simply use wc -l
cat /etc/passwd | wc -l
If you want to seek for a particular user you can do it with grep:
cat /etc/passwd | grep {username}
cat /etc/default/useradd
USE THIS CAREFULLY THERE IS NO GOING BACK
sudo userdel {username}
sudo useradd -m {username}
sudo userdel -r {username}
sudo useradd -m {username}
sudo passwd {username}
sudo useradd -r {sysuser}