I thought 3e was pretty explicit about their assumption of a 4 person base party. CR is built around that assumption.
More than four sounds right for my memory of most earlier edition module suggested party number ranges. Usually something like for 5-7 characters of levels 5-7.
Modules in older editions usually did make suggestions on party size & level, but if you look over- I walked over to my shelf and did a random grab of 1Ed/2Ed modules: 4-6, 5-7, 6-8, and no suggestion as to party size (just level).
But if you're homebrewing, there isn't anything as structured as the CR system built into the game that tells you how to tailor adventures. Its mostly feel.
However, while the CR system does assume "four fresh characters (full hit points, full spells, and equipment appropriate to their levels) (MM, p7)" it does not tell you the class breakdown of those four PCs.
DMG p48
A monster's Challenge Rating (CR) tells you the level of the party for which that monster is a good challenge. A monster of CR5 is an appropriate challenge for a group of four 5th level characters. If the characters are of higher level than the monster, they get fewer XP because the monster should be easier to defeat. Likewise, if the characters are of lower level than a monster's Challenge Rating, the PCs get a greater award.
Parties with five or more members can often take on monsters with higher CRs, and parties of three or fewer are challenged by monsters with lower CRs. The game rules account for these facts by dividing the XP earned by the number of characters in the party.
IOW, while DMG's text on CR assumes 4 PCs- and the MM assumes that they may have spells (though linguistically, it may just be using that as a general stand-in for all non-gear resources of a normal party that level- spells, powers, special abilities)- it doesn't tell us the party is made up of a Warrior, Rogue, Divine Caster and Arcanist. We just assume that this is the case.