For better or worse, combat encounters are where most of the rules for tabletop RPGs exist. This is likely because the biggest tabletop RPG was based on war games from the 1960s and 1970s.
There’s two ways to consider designing combat encounters: By the Numbers, and By the Narrative.
By the Narrative is the simpler one: it assumes that not every fight will be to the death. When somebody gets the thing they want, or it becomes too risky to keep fighting, they’ll run away.
One example is a robber who wants somebody’s money. If they’re initially told no, their intent won’t be to kill; wounding or incapacitating their victim will usually be enough to take what they want. Though if the victim proves to be sufficiently dangerous while fighting back, they may run away instead.
By the Numbers, as the name suggests, relies on using mathematics, and uses the following process.
Start by figuring out how much damage, on average, the players can do per round. Multiply that by how many rounds you want it to last. Three rounds is typically a good number. This is the total HP (or the equivalent thereof) all enemies in the combat should have
Regardless of the method you choose, it helps to add in some details for the environment; a gang of robbers in an alleyway is a different experience from fighting them in a path through the forest. “Featureless White Rooms” where character can only use their abilities is considered poor form; where the fight takes place can give more options. Alleyways are narrows, and can be blocked off if one side outnumbers the other. A road through the forest can be deviated from if somebody wants to escape; but who knows what lies in wait among the trees?
