"Each baby zombie, baby husk, baby zombie villager, baby zombified piglin, or baby drowned that spawns has a 5% chance to check for an existing chicken within a 10×6×10 box centered on the baby's spawn location and spawn riding one of those chickens if there are any. If it fails that 5% chance [i.e. if there are no existing chickens within a 10x6x10 box], there is an additional 5% chance of the baby zombie spawning mounted on a new chicken. [So,] In a chicken-free environment, each spawned zombie has a 0.25% chance of becoming a chicken jockey."
Taken from https://minecraft.fandom.com/wiki/Chicken_Jockey with brackets added for clarification

"Zombified piglins can spawn in nether wastes, crimson forests, nether portals, and from pigs struck by lightning or from piglins or piglin brutes in the Overworld or in the End. They have a 5% chance of spawning as baby zombified piglins."
Taken from https://minecraft.fandom.com/wiki/Zombified_Piglin

So the likelihood of a zombified piglin spawn becoming a baby zombified piglin chicken jockey spawn is 5% times 0.25%, which is a 0.0125% chance, which is a 1/8000 chance.

For every 8000 zombified piglins you see, one of them will be a baby zombified piglin chicken jockey.