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Hello, LynxInoubliable8614!
The idiomatic language refers to language and sentence structures that are typical of the English language. So, there is no specific number of words or phrases to use, but you need to apply it throughout the text. In this criterion, you show your ability to make your communication more colourful, engaging, and effective. You illustrate complex ideas and emotions in a concise and familiar language.
https://www.alloprof.qc.ca/fr/eleves/bv/anglais/epreuve-unique-5-eesl-a0131
Here is a list of examples: use of gerunds, tag endings, phrasal verbs. It goes beyond inserting a few expressions into the text.
https://cdn-contenu.quebec.ca/cdn-contenu/education/evaluation-epreuves-ministerielles/guides-parents/Guide_parents_ALS_Enrichi_5e_Sec.pdf
Do not hesitate to ask more questions!