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On the back of some Insane cards will have a number signifying a minimum amount of investigators in play in order for the card to be valid.
It should be the amount of players playing not investigators.
This only becomes an issue when you are playing solo with two investigators.
For example, there is an Insane card that will have "2+" on the back. Without spoiling the card, it is obvious that it only becomes a detriment with 2 or more PLAYERS.
Also, it is impossible to play with only one investigator, so the "2+" would be redundant unless it is referring to players.
Not everything is perfect that way. The investigator silhouette indicates to me that it's counting investigators and not players.
If you read the learn to play guide verbatim, it mentions that the terms player and investigator are used interchangeably from that point on. So as far as the rules are concerned, investigator = player (but obviously a solo person controlling 2 investigators is the one exception to this rule.)
So you are right, but the way the rules are written it makes sense in its own context.
RRG page 7
"Each Insane Condition has a required number of players which is indicated on the bottom-right corner on the back of the card." (emphasis added)
So in a Solo game you will only ever use 1+ labeled Insane Conditions regardless of how many investigators you're using.