As I understood it, directly after WW1 Germany started a natalist policy.

Not that I know of. The Nazis did though, starting in the 1930's.

By keeping on the fighting they could have resisted some more, but the army was desorganized and they probably would have lost more men than the Germans.

They could not resist some more. They were beaten badly within weeks. Their biggest ally, England, was driven off the continent. Only by surrendering was France able to avoid being occupied completely. And even that only lasted until 1944 when Germany decided to do it anyways.

Therefore how important was the idea of preserving the French population number in order to try to catch up with the Germans for the next war?

It would take generations to catch up to just fill the gap at the start of the conflict. Not taking into account that Germany would annex parts of pre-war France, and that the Germans themselves focussed on a higher birthrate. This seems like an impossible idea. If Vichy-France had existed longer, it would always have been a puppet state. Even if they somehow miraculously managed to outbreed the Germans, Hitler would not have them allowed to create an army that could threaten the Third Reich.

Your assumptions are wrong. The reason France and Germany were roughly equals until around WW1 isn't due to any of the countries engaging in a natalist policy. The reason France was a huge power before then and why it started to decline in relation to Germany is because it's demographics are completely different than any other European nation.

Let's say that France's population had grown at the same rate as Germany's starting in the early 19th century, its population would be 110mln individuals today. If it had kept pace with England and Wales starting in the 18th century, its population today would be about equal to the United States. France is unique in the western world in that it did not experience a population surge in the 19th and 20th century, instead, it got its bump much earlier and its birthrate started declining in the early 19th century.

Not only did is the French demographic curb different than the rest of Europe and the western world, it also experienced a far lesser drain to its population through colonization of the Americas than its rivals. France has historically had very atypical demographics for a western European country.

The French surrendering lies politically in pretty few things (I will not develop the military aspect of it as it is pretty well known and is not really part of your question). The government included defeatist politicians (not defeatist in that they were pessimistic, defeatist as in they actually advocated that a French defeat would actually be better in the long run for France). A lot of French politicians actually wanted to continue the fight despite the loss of Paris, including Reynaud the then leader of the government, the President and a lot of high ranking officers.

Reynaud though lacked political firmness and Petain got his way. Reynaud resigned and Petain got in his seat, the first thing Petain did was anouncing that France was defeated and that the fight should stop anihilating in the process the already damaged morale of the French troops, despite the fact nothing had been decided yet.

My point is no there was absolutly no thinking about a third world war. The majority of French politicians wanted to fight to the end whatever the costs and we could really imagine an alternate reality where Reynaud keeps the political upper hand and manages to continue the fight in some way (I will not comment on whether it would be a success or not as it would be pure speculation on my part).