Who does qualify as a resistance fighter is a complicated question, because it's not only a historical one: it cannot be separated from political and societal questions. I can only present the issues for the French resistance during WW2 but a comparative assessment would be interesting of course.

In the French case, nothing less than 5 laws were passed between 1945 and 1992 that defined - and redefined - the status of résistant: it was a lengthy and controversial process.

The fundamental problem is that there were two types of resistance: the "military" and the "civilian". The "military" resistance was relatively straightforward to define: the actions of the clandestine organized armed groups who had engaged in direct combat against Axis forces. Because these organizations were set up like regular military ones, with commanders, chains of command, and units, identifying who did what and when was generally possible after the war. This form of Resistance is the one usually featured in a simplistic way in popular media: Sten-wielding, rough-looking men in leather jackets killing Nazis and blowing up trains. These men (and a few women) fought not only in the French maquis before June 1944 but also on several fronts of WW2, from North Africa to Italy, and then in Germany. They were, and still are, the most visible form of the Resistance, and perceived as the most heroic.

The "civilian" resistance was, and still is, much more difficult to define. Its members were involved a large variety of clandestine actions meant to help the Allied war effort and to protect the victims of Nazi persecutions. Their resistance activities included propaganda, infiltration, logistics (false IDs, provision of food and medical assistance), transmission (radio, courriers), smuggling and hiding people, strikes, disobedience, caritative actions etc. Some of these actions were carried out by members of Resistance networks, but others were done by isolated individuals. This form of Resistance, shadowy by nature, was much less spectacular and carried out by people who did not look like fighters. Some of them may not even have considered themselves to be part of the Resistance unless they were directly affiliated to a specific organization.

In Hilary Kaiser's oral history of French "war brides" who married GIs after the war, one of the witnesses tells how she helped people cross the Demarcation Line, including a couple of Jewish friends, so that they could go to the Free Zone. She did that eight times, apparently for the thrill of it ("All of us young people were crazy. We wanted to do so many extraordinary things"). She does not seem to have thought herself as a Resistance member, and she would not have been able to claim a Resistant status.

In the immediate postwar, the focus was more on the military resistance than on the civilian one. To get an official Resistant status (which opened rights to certain benefits), the candidate had to prove that they had belonged to an officially recognized Resistance network for at least 90 days before the 6 June 1944 (it was also possible but more difficult for isolated people). And one had only 9 months to apply for the status! The strict requirements had partly a political basis: at a time when WW1 veterans were still an important and vocal constituency, the requirements for being recognized as a WW2 resistance fighter had to be aligned on those used for WW1 vets, notably the minimum period of 90 days. And there was also budgetary reasons to limit the number of pensioners holding the status of Resistance fighter... The date of 6 June 1944 was controversial: some thought that the 6 June date limit was unfair while others considered that a person should have known by early 1944 on what side of the fence they stood. The short deadline for the application was another issue.

Gaullists favoured the "military" perception of the Resistance while French Communists (and sometimes other parties) preferred a tentpole approach more inclusive of civilians, as they saw the Resistance as a general popular movement. There was also a debate on what made a person a "resister": their actions or the risks they took? Under the more stringent "military" perception of the Resistance, an isolated individual who had hidden Allied aviators, Jews, or Resistance fighters, was not a member of the Resistance, even though this person risked deportation, torture, or death, just like the fighter who shot Nazis, planted bombs and derailed trains.

The following decades saw changes in the legislation, some more liberal, some more stringent, though the deadline for applying to Resistant status was regularly postponed. In practice, obtaining the card of Combattant Volontaire de la Résistance (voluntary Resistance fighter, CVR) was more difficult for people who had not been part of established resistance organizations, as the application required the testimonies of two witnesses with official Resistance credentials. Eventually, the CVR card was granted to about 260,000 people (Lacour-Astol, 2015).

It was only in the 1970s that the perception of the Resistance as a military action changed to one that saw it as a moral opposition to totalitarism, and thus theoretically inclusive of all forms of resistance, military or civilian (Wieviorka, 1996). I've written here about the situation of women in the Resistance: traditional gender roles applied, and they were used mostly in "supportive" and "nurturing" roles, and resisted at home. After the war, few claimed the Resistance status, perhaps because they did not feel they deserved it, or because they did not want to upstage the "true" fighters.

For those reasons, it is not possible to provide a definitive number of French Resistance members: there are many ways to count them and historians cannot revise the way applications to Resistance status were granted after 1945. The database of Resistance titles maintained by the Historical Service of the French Ministry of Defense (Service Historique de la Défense) contains the names of 610,000 people who petitioned for a title related to Resistance activities: 443,000 of these petitions were granted, so that's a baseline number. However, the service is careful to include the following caveat:

The aim of this corpus is not to draw up a list of all Resistance fighters. Indeed, many people who fought against the occupying forces during the Second World War never made themselves known. Moreover, some forms of resistance action did not give rise to administrative procedures leading to the creation of an individual file.

Sources

• Kaiser, Hilary. French War Brides in America: An Oral History. Praeger Publishers, 2008. https://books.google.com/books?id=VyFnAAAAMAAJ.
• Lacour-Astol, Catherine. ‘Obtenir le statut de combattant(e) volontaire de la Résistance’. In Le genre de la Résistance, 239–74. Académique. Paris: Presses de Sciences Po, 2015. https://www.cairn.info/le-genre-de-la-resistance--9782724617009-p-239.htm.
• Wieviorka, Olivier. ‘Les avatars du statut de résistant en France (1945-1992)’. Vingtième Siècle. Revue d’histoire 50, no. 1 (1996): 55–66. https://doi.org/10.3406/xxs.1996.3520.