That's not how it works in France at all. You get prosecuted on all charges, but only condemned for the most grievous offense. So for a murder+assault+trespassing case, if found guilty of all charges, your sentence would only be for murder.
As all condemnation it takes context into account, like the assault + trespassing, but if there were limits on murder sentences then you'd only be sentence-able to that crime's legal sentencing limits (there are none for murder, which is the only crime that has no sentencing limits).
It's the maximum legal sentence, and 20 years is absolutely no joke