This is just a new compilation of all the additions in the old hardcoded list. People kept adding new discoveries since 2007, and now the old list spans over 8 pages. And since the OP is sadly inactive, I figured it would be much easier for users to search for their needs if it is in one page.
Credits go to all the contributors of the old thread! (I have included some names, I don't have the rest)
Note: If I cannot understand a post, I just copy-paste it here. Some of these are merely copy-pastes.
Maps, Regions, Landmasses, Resources, Climates, Vegetation:
*Number of regions:
200 (Including sea regions)
*Map size smaller maps (e.g map_regions):
510x510 pixels -
wilddog
*Map size bigger maps (e.g map_heights):
1021x1021 pixels
(same as above)
*Hidden Resources:
64
*Limit for the number of times you may use a resource:
186 -
Werety
*Maximum amount of land masses a map can have:
20 -Razor, B.Ward.
*Maximum number of climates:
12
*Maximum number of vegetation models per vegetation group:
4
*Adding a new ground_type is
impossible as it is
hardcoded.
*In descr_model_strat you can not get more than
256 "type".
*Entries in descr_rebel_factions: there is
no limit of creating, the limit is using them, as you can have
only one per region.
Buildings:
*Maximum number of buildings per tree is 9
*Maximum number of building trees is 128.
*The highest base cost a building can have: 65,535 florins
*Required buildings in EDB:
--core_castle_building
--core_building
--port
--castle_port
Units, EDU, Projectiles, BMDB:
*Maximum number of units per mod/game (EDU): +/- 500
*Max number of units for a faction available in custom battle selection screen: 200
*Maximum ammo limit:100 ... Rocket Launcher salvoes is limited to 3 only.
*Total number of armour_ug_levels: 7. On battle map, the levels are counted as 8. -Tellos Athenaios
*armour_ug_levels can go up to where you want. But armour_ug_models can be used only upto 4, one original and 3 upgrade models. -Ishan
*The current known highest number of entries for the model.db: "the third age have 1659" -Hospitallers Kane
*The apparent limit of explosive projectiles (exploding_[name]_shot) in descr_sm_projectiles: 9 -Gigantus
Campaign, Strat map data:
*Faction Limit: 31 (Including slave/rebel) ((The 1 is the slave/rebel faction))
*Maximum number of resources: 27 -Nakharar
*Maximum number of cultures: 7
*Maximum number of religions: 10
*Maximum number of crusades at once: 1
*Can edit the frequency of crusades: No
*Can Orthodox be modded as a crusade target: No
Named Characters, Agents:
*Maximum number of traits that can be triggered: 11
*The max number of letters for a character's name: 42
*Agent types: 12
*Limit on the number of Levels a character type can have: 10 (note: level 0 is also counted) -Kiliç Alì
*In the descr_character file, you can only define 30 different entries in one particular agent type (does not include generals / captains, but does include spies / assasins at the least and probably others as well. If you type in the 31st entry it will go ballastic. -RollingWave
*Maximum number of Ancillaries: 8
Sounds and Music:
*Number of sounds, samples and musics: Unlimited
*Number of accent groups: Unlimited
If you find a new discovery or a hardcoded limit that is not mentioned here, post it, and I'll update the list.
This is for the good of the modding community. It is better to have 1 single list that contains all the limitations of the RTW-M2TW engine than having to make a thread every time somebody runs into a problem.