"Thank you very much!"
"tusen" = thousand
"tusen takk" = thank you very much or thousand thanks
"Takk så mye!" and "Mange takk!" are both natural and good translations, imo.
I use both of them, and the above, in slightly different settings and around different people. They are perhaps a bit more common to the older generations, and less heard among teens.
Either way, nobody's going to be offended that you thanked them in the "wrong" way, the gratitude shows on your face! :)