Whenever I sit down to generate game content for play I make a conscious decision of how much stuff to cram in one avenue of play. Elements in an encounter, session, adventure arc, or campaign are all intentionally spaced out to ensure its all playable and fun at the table. I don't want fifty bajillion things to track at any given time, nor do I want players stuck dealing boring one dimensional adventures.
I try to have about 5 things. A magic number based on cognitive power of humans. You can go a little more, some people have a little less, but this will be perfect for most players to know whats going on, and for me as a GM to remember off the top of my head. By keeping about 5 moving pieces (sometimes a little more, sometimes a little less) I can keep it all in my head and so can players as long as they don't invent too many red herrings through faulty reasoning. That simple, end of article right?
Well, depends, whats a thing?
A thing depends on the context.
For an encounter its participants, terrain, consumable inventory, rival goals, and the like. If I have 12 identically armed bandits, that is "one thing". If 6 are archers and 6 are spearmen, that is "two things". If there are 6 spearmen fighting 3 swordsmen and 3 archers that is "four things". The swordsmen, the spearmen, the archers, and the conflict between the two groups. If the terrain is interesting at all, you are already up to five things. If the PC's are on a mission with limited supplies, their own goal of whether or not to get involved is a sixth "thing". This is something I try to build into random encounter tables.
Move up to a session or to a based location and you have things such as factions, large monsters, major treasures, puzzles and the like. Minor things like a locked desk drawer with a poison needle trap and 10 gold, not really a "thing" in this context. The room which contains an orc shaman, the trapped desk, and a bunch of scrolls and books with important investigative clues to the complex as a whole would be a "thing". The room as a piece of the whole is a "thing". A series of small guard rooms full of goblin thieves with a faction goal would be a "thing". The giant spider prowling the halls would be a thing. The rumoured tomb of a wight with a magical sword is another "thing". The place is in swamp that has partially flooded the compound is a fifth thing (the terrain). That site is risking being overfull. If I also added in a rival adventuring band that is six (still fine), and a dragon that roams the swamp hunting stragglers its at seven (pushing it), and then threw in a cult of Gulnor the Frog Demon who are trying to turn the complex into a temple...its now a big mess for most folks (though, 7 +/-2 means some folk can still handle it).
But what about mega dungeons? A mega dungeon is meant to be a multi-session longer term game. So I treat that similar to an adventure arc. In an arc each adventure location is a "thing", any overarching goal is also a "thing" . For example, if we looked at a a totally unique game concept where players arrive in the town of Bistram to look for a prince who was kidnapped by demons and held in the ruins under an old Cathedral, that campaign (as a megadungeon) would have a few different elements. Rescue the prince, the sub-dungeon of the cathedral basement, the sub-dungeon of the catacombs, the sub-dungeon of the natural caves, the sub-dungeon of the demonic palace. If the town of Bistram had issues to solve at the same time you'd be pushing up to six "things".
As another non-megadungon example you might have a quest to overthrow a corrupt baron. You have the "things" of dealing with the corrupt baron, three site based adventures (freeing prisoners from his jail, allying with or defeating the forest bandits, maybe a monster he won't deal with that threatens peasants), and you then have room to add in another complication or side quest, perhaps something like an invading army of the undead that prevents you from just obliterating his forces but requires you to keep them intact for that future purpose.
Full fledged settings or campaigns then step it up a level. Each megadungeon or adventure arc is a "thing". An overall theme or goal is a "thing", examples being carving your own Kingdom out of the region or say being British explorers looking for the Eye of Set. Large scale faction conflicts are a "thing", such as an occupation of the region by a foreign power, or a war with the dark lord and his orc hordes.
If you have a campaign about interpersonal relationships, each of those relationships often becomes an additional "thing" at each level. If there is a bitter blood feud or devotional love between two PC's that becomes an important element at the encounter, session, adventure arc, So when figuring out content for your own game, keep that in mind when writing an adventure.
As a final note, if you are a fan of red herrings and twists, depending on how much time you want players wasting you may or may not want to consider those things. If its an established part of game play to sift through red herrings and expect twists, might not be a "thing". Much like an M.Night Shyamalan movie, its expected. If these are unusual parts of your game, you might want to consider them a "thing" because players will spend a lot of time and attention on them over other elements.
TL;DR : 5 elements
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