This is also something that depends heavily on regulations. In my home country, invoice numbers have to be sequential by law, although you can restart the numbering every year.
Yes, even if it's not a legal requirement it's definitely best practice to have sequential invoice numbers. I thought about this at the time but these numbers aren't invoice numbers, only order numbers.
A sequence per "series", where a series can be a fiscal year, a department or category, etc. But I am not sure if you can have one series per customer, I only find conflicting information.
You can have more details here, in the section "Complete invoice":
