What is the default Hybrid Culture Frequency game rule in CK3?

Default: The default amount of Hybrid Cultures will appear. (patch 1.20.1.2)

Does changing the Hybrid Culture Frequency rule disable achievements?

No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · AI · Culture

Hybrid Culture Frequency

Default
Less Common
None

Hybrid Culture Frequency is a game rule in Crusader Kings III (AI and Culture) with 3 settings: Default (the default), Less Common and None. None of its settings disables achievements. Patch 1.20.1.2, from game/common/game_rules/00_game_rules.txt.

This section as JSON: /raw/game-rules.json

Quick answers (2)

What is the default Hybrid Culture Frequency game rule in CK3?: Default: The default amount of Hybrid Cultures will appear. (patch 1.20.1.2)
Does changing the Hybrid Culture Frequency rule disable achievements?: No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

3 settings

Default
Default
The default amount of Hybrid Cultures will appear.
default_hybrid_culture_ai_frequency
Less Common
Fewer Hybrid Cultures will appear.
The script checks it 5 times, in 3 places
Decisions 1
- Blend … and … Traditions
Events 1
- Cultural Intermingling
Character interactions 1
- ai_cultural_hybridization_interaction
Every has_game_rule = less_common_hybrid_culture_ai_frequency in game/common and game/events, by the top-level object it sits in.
less_common_hybrid_culture_ai_frequency
None
No non-player created Hybrid Cultures will appear.
The script checks it once, in 1 place
Scripted triggers 1
- game_rule_can_create_hybird_culture_trigger
Every has_game_rule = none_hybrid_culture_ai_frequency in game/common and game/events, by the top-level object it sits in.
none_hybrid_culture_ai_frequency

Source: game/common/game_rules/00_game_rules.txt line 597 (key hybrid_culture_ai_frequency), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.