Some numbers have multiple pronunciations. '7' sounds 'なな' and 'しち' etc.
Somebody wrote there is a website about Japanese number.
Now "nanatsu no taizai" makes sense XD
Though I’m not sure 'pieces' suits which English.
It is not '七つ切れ' but '七切れ'.
Go for it! (this is my favorite phrase that I have learned recently. And to you.)
www That's not really how you use "go for it" ww
I want to say
I was wondering why the translation was seven pieces. It sounded weird to me too .