I was wondering if it would be possible, by adding more core set into the mix, to play the game with more than 4 investigators...
(More core sets would be require to create the decks, so there would be enough duplicates for the Encounter deck as well.)

I've tried to find a similar topic in the forum, but couldn't find any, yet.

That should indeed be possible I guess with enough core sets.

Tho they have a max player number for a reason, as the scaling per scenario might get out of balance.

On the other hand, if you have the players and the core sets, why not just try?

It's your house, your table and this is a co-op game, so you can add an extra player if you like.

Also, you don't need extra encouter decks if you try this. Just the one you would always use.

I think it depends on what scale we're talking about. Could you run with 6? Yeah, definitely. Could you run with 12? Yeah, but I think you'll find there'll be people with nothing to do. Also I think the higher you go on players the more it bloats playtime. Still, if you're trying to shoehorn a few extra people from your group in then I think it's OK.

You could also maybe have couples run an individual investigator jointly. Just a thought.

Some people have tried 5 investigators with 2 core sets, they didn't duplicate the encounter sets and it seemed to work just fine, although it did raise the difficulty a bit (and the game took a bit longer). The player decks weren't perfect, as they were short some neutral cards, but once Dunwich Legacy comes out you should have enough cards to not need to worry about that.

I'd hesitate to go much higher than 5 players because, as others have said, the amount of time it takes to play the game. I definitely would not go over 8. Also, you'd probably need to come up with some kind of scaling level of doom since doom will go up faster with more players. I'm intrigued by the idea of doubling the encounter deck with 6-8 players, but I think you'd still be better served to just shuffle in the discard pile when you run out. That way you don't end up with 2 of creatures that should be unique. If you really wanted to double, I wouldn't double the scenario-specific deck (like Icy Ghouls and other cards from "The Gathering"), just the other sets (like Ghouls, Rats, etc), but that might change the flavor of the game too. Definitely do not double any set aside cards.

While I could see the appeal of wanting to play a game with 5-8 (or even more players) if that's the only gaming opportunity you get with friends, but you might want to just consider splitting into two groups and playing it that way. Make it a competition to see which group gets the better ending, compare stats, etc, if you wish.

There's nothing special happening when the encounter deck's discard pile is shuffled back in, so there's no technical difference if you double the cards. If you only double the non-uniques the uniques show up more rarely, which could be a way to lower the difficulty.

The only practical problem I see is fitting all those investigators in the same location in an 8-player game

Do what SuperMarino said - play two tables. Survivors from each, if any, get to play with each other next game.

If you're playing that many people I'd almost just suggest getting the Arkham Horror regular board game instead.

Oh, been there, done that.

I have the full collection of the board game.

This LCG seems a better coop game + easier rules (and setup), faster gameplay with a small spice of RPG flavor as well

If you're playing that many people I'd almost just suggest getting the Arkham Horror regular board game instead.

There's nothing special happening when the encounter deck's discard pile is shuffled back in, so there's no technical difference if you double the cards. If you only double the non-uniques the uniques show up more rarely, which could be a way to lower the difficulty.

The only practical problem I see is fitting all those investigators in the same location in an 8-player game

Do what SuperMarino said - play two tables. Survivors from each, if any, get to play with each other next game.

There is a difference: some enemies are worth victory points, so if you put more of them in the deck, you increase the potential XP (since each player gains XP for defeated enemies). On the other hand, as you said, not doubling them reduces the probability of drawing them. Also, it makes absolutely no sense to double the locations, the agendas or the acts (they're all part of the scenario-specific encounter set).