I can run four characters solo with Pathfinder 1e. I tried running six, but I started to feel that stress on the brain, and I found two of the characters began to get ignored every turn as I focused on the front four. Even with five, one inevitably did not have much to do, and it was often because of not having enough front-line space in tight dungeon corridors.
Thematically and story-wise, four always felt better and what the game was designed for.
I would rather play with fewer characters and scale encounters down than create a lot of work for myself and have to scale encounters up. For a solo game? Get the flavor of the encounter and story, and keep your life simple. Many adventures are designed for four characters, so I do not have a problem managing this, and it works well.
Pathfinder 2e Solo?
I struggled with Pathfinder 2, I will admit it. The action economy and the tight game tuning mean you need to pay attention and make good choices every turn. That action economy will likely be different for different classes and builds. Since you have three actions per character, a party of four means up to twelve decisions for your four solo characters every turn.
Yes, in a Pathfinder 1e game, you have your three actions, but the types of each one are strictly defined:
In a normal round, you can perform a standard action and a move action, or you can perform a full-round action. You can also perform one swift action and one or more free actions. You can always take a move action in place of a standard action.
So most of the time in Pathfinder 1e, one attack per turn and one (optional) move are safe to assume. If you are on the front lines, ignore the movement. For many characters, a turn means just one attack and moving on to the next character; for four to six decisions a turn, if a few characters in the back ranks shift or take swift actions.
With Pathfinder 2e, I see many people stress taking those actions, and being smart about them means the difference between success and failure. So you need to make all twelve per turn for a party of four, and you need to make sure you are using them wisely. Yes, some actions take multiple actions, but I would rather have time to consider options than tick through a dozen per party turn.
That feels like a lot for me, especially since I started to zone out with a party of six Pathfinder 1e characters and begin to ignore two of them. I have this feeling most turns two characters will likely be playing sub-optimally.
Solo Play as the Duo
I am likely playing Pathfinder 2e as a duo. I have no problem cutting an encounter in half in terms of numbers or solo monster hit points and focusing on a dynamic duo that learns to work together. That feels good to me, and it cuts down on the overload I feel with Pathfinder 2 solo games. This caps my actions at six per turn, making this task manageable and matching the per-turn average I was handling with Pathfinder 1e games.
This is like your typical "buddy cop" movie, like having a cranky dwarf and arrogant elf learn to get along and work with each other. This also avoids one falling unconscious and the adventure coming to an immediate end, as one could help the other.
At most, I would have three work together, but I would keep my party size to two since there will be times the duo gets an NPC for a while to assist in the fight. If you put your max at three, that NPC raises it to four, and you are back to a heavy action load to manage each turn.
Also, I may adjust the bonuses given by actions that provide party modifiers. There is a vast difference between giving four party members a +1 to a turn's actions versus just two. If I play duos, I am possibly increasing those bonuses to +2 with parties of one or two and +1 for three or more - and calling this the Buddy Rule. We ran into this problem with D&D 4E as well, and this is something I wish I would have done back then for small party sizes.
Plus, with a duo, with every action you take, you are asking this one question, "Do I help myself, or do I help my partner?" Forcing yourself to ask this question helps enforce the "team play" focus of many actions, and you would not get these options if you were playing with one character alone. Since teamwork is highlighted in the game, if you play a single character, many choices you can make with the action economy (and many powers) will be pointless since there is no one else to assist.
Also, in that two-person theme, the entire buddy-cop "helping each other" story of the game advances every time you make choices that force them to work together and realize they are better with each other than they are alone.