I don't think it's really quite a "Vs" kind of balance on Hope and Fear, like they're kind of independent factors (despite the same roll being the theoretically primary generator for both), but I think more players will change them in a few ways. Like:
1) More players means it's more likely a PC will be able to do the "Get 2 Hope on a Short Rest" thing, because it's more likely they'll avoid feeling they need to use both Rest Actions to recover HP/Stress/Armour Slots. I think that's fine but it might mean PCs going into encounters with more Hope (probably not a big deal).
2) The total max Hope in the party will be higher - 36 with 6 players as opposed to 24 with 4, but your total max Fear will remain the same at 12 (I think!). Each PC starts with 2 Hope too, so there will be more around.
But the action economy, because it's not turn-based nor completely arbitrary (unlike some PtbA games for example), is a bit different to most RPGs - PCs who take more actions get more Hope and generate more Fear - and the de facto result - I think - with a bigger party will be the PCs able to absorb a lot of Hope, and choose when to spend it (assuming the spotlight is bouncing around a lot), whereas you'll be frantically spending Fear as they generate it to try and avoid capping out.
3) Tag-teams - 6 PCs means they can pull of 6 Tag-team attacks per session - that seems obvious but I think do be ready for any tough NPCs to get absolutely blown up by this if the players notice and don't like, spread out their Tag-teams. I guess any pivotal encounters may need really serious multi-stage bosses if you don't want them to get absolutely pounded. Esp. as the way tag-teams work is the PC get to pick which of the rolls to use, so are more likely to pick a "with Hope" roll (for obvious reasons), which means they get to keep going - so potentially you might see a tag-team into tag-team into tag-team situation if a the PCs have a lot of Hope (which I think quite easily could happen). How to deal with this? Probably try and encourage them to blow their 1 per SESSION Tag-teams early. I do think I am going to see a cool major boss just < snaps fingers > vanish to this sooner or later when they get wise lol. As Tag teams combine damage and pick the most favourable roll usually what happens is they manage to cross the severe damage threshold by a country mile and also get 1 Hope each and also can keep going because they didn't fail or roll with Fear (I guess in theory both rolls could be fails or with Fear, I don't know the odds on that - I didn't see it happen personally).
Probably more but my caffeine is running out and it's now too late in the day to drink more!