People say that the game is easiest to win using only one investigator. The game adjusts the rules depending on how many invests there are. However, there is one area in which the game apparently never adjusts: Amount of seals needed for victory. Unless I am over-looking something, players always need to seal 6 gates to win, even if you are using only 1 or 2 investigators. I prefer controlling 2 invests in my solo games, but this seems to reduce my chances of a seal win. Am I correct in thinking this? Why doesn't the number of seals for victory reduce as the amount of invests reduces?

Because closing gates victory is a lot easier with 1-2. Turn 1 wins with 1-2 investigator should happen from time to time, not very likely with 4+ investigators.

With just the base game you should eventually be able to get a sealing win around 70-80% of the time with two investigators, as you figure out the strategies. To start with, however, expect your success rate to be considerably lower. It took us quite a few games just to get our sealing rate up to 50%.

With expansions (gate bursts, double-doom cards) this rate goes down, but we can still get a sealing rate just under 50% with any combination of small-box expansions. This is after a couple of hundred games, though - it really takes practice, and we still make stupid errors that cost us the game sometimes.

Are you overlooking the fact that one Investigator alone vs. an Ancient One only needs successes equal to the number of Doom Tokens? I know many hardcore players like to call this a "draw"...but maybe the people that win with only one Investigator count an Ancient One Beatdown as a "win"...like the manual says it is. If you only consider "Seal Wins" as wins, you should make sure the people who report their solo Investigator wins are doing the same.

I've never won a game playing single player but then again I always play Azothoth.

I play a lot of two player, two investigator games. I've found that closing victories are far too easy with that set up, and you just need a little bit of luck to pull them off. I managed to beat Atlach-Nacha that way fairly handily.

To avoid this, we play with the following rule:

"For a closing victory, you must close all gates on the board and have a number of gate tokens equal to either the number of players or four, whichever is more."