Question: "Can I add more tenets to historical/existing religion"

Answer: Yes, with console or other mod.

This mod doesn’t change existing religion. I leave that to other mods, because I don't want to force such changes to all players, but it is easy to include more tenets with mod or console.

Option 1: With command console:

run CK3 in debug mode (-debug_mode) and use console command (console open with `)
run command:
add_doctrine tenet_XXXX

for example
add_doctrine tenet_gnosticism

list of doctrines (tenets) you can find in Crusader Kings III\game\common\religion\doctrines\00_core_tenets.txt

Works for existing saves.

Option 2: New mod:

create mod with changes in common\religion\religions\00_XXXX.txt (for example 00_christianity.txt)

in section for given faith e.g. (faiths => catholic) add new lines

#Tenets
doctrine = tenet_communion
doctrine = tenet_armed_pilgrimages
doctrine = tenet_monasticism
doctrine = tenet_XXXXX ....

I'm not sure about this method save compatibility. Could need fresh start.

Question: I would like to have X slot instead of 20.

Answer: To change number of slots please go to Steam folder and open:

Steam\steamapps\workshop\content\1158310\2218249327\common\religion\doctrines\00_core_tenets.txt

and change line

number_of_picks = X

This way you can set any number of slot, but please take into account that there is “only“ 20 blank slots available, so adding more than 20 slot will force to use not empty tenets.

As of now I do not know how to make it configurable via games rules or in-game decisions. It looks like constant/static value. If anyone know how to make it dynamic (so it can be changed without editing file) please let me know, I would be glad to include it in mod.