You should base the number of points of the lowest number of players you have regularly. As an example: if it usually varies between 3 and 5 and it is 2 or 6 once in a blue moon, you should build the charracters as if you had a group of 3 players.

You really need to ask yourself "How often will there be only 2 players?" "How often will there be three or fewer players" etc.

On page 12 of Mutant City Blues, another gumshoe game, if found this: "Players who can only attend every now and then get the same number of investigative build points as everyone else, but are not counted toward the total when deciding how many points to allocate."

the way I interpret this, you should base the number of points of the lowest number of players you have regularly. As an example: if it usually varies between 3 and 5 and it is 2 or 6 once in a blue moon, you should build the charracters as if you had a group of 3 players.

You really need to ask yourself "How often will there be only 2 players?" "How often will there be three or fewer players" etc.