|Darksol the Painbringer|
If you have encounters with multiple enemies, reduce the enemy count appropriately. If an encounter throws 4 level-1 enemies at the party, make it 3 or 2, depending on how many players are in the party.
With a smaller party, maybe shrink the arena size down to make enemies a bigger threat and to give the same space value as with a normal-sized party. If it's a 30x30 room, make it a 25x25 or 20x20 room instead. Since there will be less enemies and players in combat, you won't need as much space to reduce the viability of getting enemies and allies in the fight. With certain fights and areas, though, it might be wise to keep the arena sizes as-is or to even expand them, to promote certain tactics like hit-and-run or kiting.
If you have encounters with singular, stronger enemies, apply the Weak template to them. This reduces their HP value as well as all proficiency-based values so that the heroes have a bit of an easier time. Feel free to throw in a couple "minion" types as well if the PCs are stomping the bad guy.
The biggest thing is that with a smaller party, there is less action economy in their favor, meaning making efforts to either reduce enemy action economy to compensate or making the PC's action economy matter more per PC will help maintain the balance the game expects.
Three players: apply the weak adjustment to all creatures you encounter.
Two players: apply the weak template to all NPCs and also apply the Elite adjustment to your players.
Just you and 1 player: Give the player 3 extra levels. So for example, a level 1 adventure is appropriately tuned for a level 4 character. Consider also giving dual class, but that's not totally necessary.
Just you and 1 player: Give the player 3 extra levels. So for example, a level 1 adventure is appropriately tuned for a level 4 character. Consider also giving dual class, but that's not totally necessary. This was how I ran the beginner box, as info. Worked fine with a homebrew Drifter I was testing for another poster.
I started with removing 1-2 combatants but I didn't like how few enemies there usually were doing that and weak adjustment for every enemy is a ton of work.
Adjusting the health down according to the weak template is quick and worked quite well for a 3 person party. They did get free archetype to be a little stronger however.