Numbering & level of revealed Dunwich Legacy cards, combined with the total number of player cards in the expansion, strongly suggest that each class will get three level 0 cards (2 copies each) and one card of level 1 or higher (2 copies). For neutral, it's unclear how many non-weakness cards there will be, but it looks to be between zero and two.
Given this, you could make full legal decks for 3 investigators using one core set and the Dunwich Legacy. You could have:
Core set investigator #1
10 core set cards for class A
2 Dunwich Legacy cards for class A
10 core set cards for class B
2 Dunwich Legacy cards for class B
6 core set neutral cards
Core set investigator #2
10 core set cards for class C
2 Dunwich Legacy cards for class C
10 core set cards for class D
2 Dunwich Legacy cards for class D
6 core set neutral cards
Dunwich Legacy investigator #1
10 core set cards for class E
6 Dunwich Legacy cards for class E
5 Dunwich Legacy cards of classes A-D
9 core set neutral cards
There's a little bit of flexibility there:
The two core set investigators could have up to 6 more Dunwich Legacy class cards, dropping either core set class cards, or neutral cards. There's additional flexibility here if there are any Dunwich Legacy non-weakness neutral level 0 cards
If the core set investigators did drop core set class cards for neutral cards, then the Dunwich Legacy investigator would be free to pick from those core set class cards for his up to 5 level 0 cards of any class
The Dunwich Legacy investigator could drop core set or Dunwich Legacy class cards in favour of neutral cards, again if there are Dunwich Legacy non-weakness neutral level 0 cards, or if core set investigators had dropped neutral cards
Obviously the whole thing is a moveable feast when describing what cards an investigator can be pick, based on what cards other investigators picked/did not pick, but those are the main flexibilities I think. In any case, the three investigators will have a minimum of 102 cards (if no level 0 neutral cards in Dunwich Legacy) and possibly up to 106 (if two different neutral cards in Dunwich Legacy) from which to pick their total of 90 cards, which doesn't leave a great deal of wiggle room.
And, each Mythos pack appears to contain one card of level 0 and one of level 1 or above per class, plus neutrals (could be one level 0 and one level 1 or above as well, but not clear), so as those are released, choices increase.
Hope this helps (I'm 95% sure it's correct).