He committed the single worst gun massacre in world history and only gets 21 years?
Talk about a Karma Houdini
That's the max in Norway.
Is it the max per charge, or max total? In other words, if they charged him with 77 counts of murder, could he get 77 21-year sentences?
Nope. Norway, along with much of Europe, practises concurrent sentencing. This means that a person can be convicted of seventeen counts of murder, each sentence attracting 21 years, but will serve all of these sentences simultaneously. So basically no matter how many murders you commit you'll only be in prison for the length of the longest sentence. If you commit a double murder, getting a 17 year sentence and a 30 year sentence, you'll serve 30 years rather than 47.
However, in some cases you can be imprisoned for life. I know in the UK the 'Moors Murderers' Myra Hindley and Ian Brady were sentenced to life and will die in prison (Hindley died at Her Majesty's Prison Highpoint in 2002 and Brady is in a secure psychiatric hospital). However, if it is true that Norwegian maximum sentence is 21 years then unless they change the law just for him he may well get out in that amount of time.