As an European federalist, I wish the answer will be "none"- Le Pen has a pretty low ceiling and it's not a matter of winning a few more votes in Pennsylvania- her polling numbers for the second round are horrible and she's trailing Macron by 30 (!) points. And considering how she didn't win any regional elections in 2015 despite finishing in first place in many of them in the first round, it's not just a mere hypothesis or a wishful "it cannot happen". It's obvious that the vast majority of the voters would rather have literally anyone before a Le Pen, and Jean-Marie knows it well (see 2002). And the most likely winner right now, Emmanuelle Macron, is a pro-European, soo... Oh, and btw, polls in France show that only 25% of the population has a positive opinion of Trump- a number that's eerily similar to Le Pen's support. Again, the more outrageous nonsense Trump does, the more toxic their connection will become.
What I do see likely is that the EU might end up splitting into "two roads", like with the Euro: a more "federalist" group of the "core" countries that actually want more deeper integration and less "national identity politics", and an "outer Europe" of more reluctant countries. But without the EU, Europe is a politically irrelevant, economically fragmented bunch of rump states with no future other than conquest by Russia or the Chinese or our own destruction through more of the wars our nationalist Governments started every few decades for centuries and centuries and centuries.
Of course, I'm talking from Spain, a country where our far right hasn't yet managed to get 1% of the vote despite being the 10th country in the world with more immigrants, and where ample majorities are pro-European. I guess we had too much of far-right fascist nationalists to want to try that ever again. Probably Germany knows that too.