Not sure about France, but next door in Germany we try to put multiple elections on the same date, because organizing the polling stations is always the most ex

Not sure about France, but next door in Germany we try to put multiple elections on the same date, because organizing the polling stations is always the most expensive thing. So it's not uncommon to have some of city mayor, city council, regional, state and federal and European elections fall onto the same date. I think the "worst" I've seen as a volunteer was 4 elections on the same day. Counting process starts by splitting the ballots (if we have all of them in one urn), and then counting and tallying them one election at a time. The ballots are distinguished by different colored paper. One voting district is usually around ~1000 voters, out of those, before Covid ~10-20% did mail-in-voting, during Covid it was ~40-50%. Then with a normal voter turnup of around 60-70% depending on the elections that are held, we end up with a few hundred ballots to count per election, with a staff of normally 8 people... Polling stations close at 6pm, and the latest I have left the voting station after counting was 8:30pm.