French here. The exact amount is negotiated (aka decided by the employer) and written in the contract. I had multiple work contracts where the indemnity would have been 30% of my annual salary, never more (which is actually pretty bad because you still have to find a job without breaching the non-complete).

Non-compete clauses must also be limited in scope (geography and profession) and must not stop employees to live from their trade. As a result companies rarely exercise non-compete clauses since they are regularly thrown away by courts ("conseil des prud'hommes" in french).

Still they are part of the so called "standard work countract". I imagine most companies just get their base contract copy-pasted from the same template.