I suppose these are Khmer apostles. Just read carefully their bonuses and you'll know, why they're swimming in faith.
From Civ VI wiki:
Grand Barays: +3
Faith and +1
Amenity for
cities with an
Aqueduct. +2
Food for
Farms adjacent to an Aqueduct.
Jayavarman VII's unique
agenda is called
An End to Suffering. He likes civilizations that have
Holy Sites and a high average
city Population, and dislikes civs that are lacking in either of these areas.
I see they have quite a few cities. Check how many of them have Holy Sites and Aqueducts. I bet, quite a lot all of them.
Their Prasat which replaces a Temple give their missionaries Martyr promotion. Check in the deal negotiation screen how many relics they have. I bet, almost all of them.
That is already huge faith output.
How many city states are religious in your game and how many envoys do they have in them?
Check their religion - what pantheon they have and what enhancer beliefs?
What is their government - probably Theocracy?
See what Scripture and Simultaneum policy cards can do.
With so many cities as I can see here and the game date being what it is, no wonder that Jayavarman has a bit of faith to spare for a few dozens of apostles and then some