I am not entirely sure, I hardly know all of the groups, But I am guessing that the 8eme and maybe also the 9eme are some of the largest groups.
Imperial guard is reenacted by dozens of groups world wide (If you can count Imperial Guard as one group anyway). So theoretically there should be the most reenactors doing the imperial guard.
I think world-wide the 21e is one of the most reenacted regiments, I believe there is a group in the UK, 2 in the US, 1 in Australia, 1 in Russia.
I am guessing that the 95th Rifles are also very well reenacted, with groups world wide, I know at least of 3 groups.
But then again the 85eme also has 3 Groups... UK, NL and my own, which is European/German.
There are also at least two 18e, one in Poland/Czech Republic and one in France.
Problem with the French regiments is, that they are not strictly Line Infantry or Grenadiers or Light Infantry. Each regiment is made out of 3 parts. Fusilier, Grenadier and Voltigeur. So you naturally end up with one group reenacting the Voltigeurs and another group the Grenadiers (As is the case with the 18e, for example)