Is there a limit for duplicated keys? How many can be linked to one source key?

No. A source key can have an unlimited number of linked keys pointing to it, so one string can feed translations to as many duplicates as your project needs.

The only structural rule is that a linked key can’t itself become a content source, so chains aren’t possible. Every link always points straight back to its original source key.

Read more about Duplicity Linking.