What is the starting value of saturation?

According to the Minecraft Wiki:

foodSaturationLevel: The player's current saturation level, which determines how fast the hunger level depletes and is controlled by the kinds of food the player has eaten. Its maximum value always equals foodLevel's value and decreases with the hunger level. Its initial value on world creation is 5.

(bolding mine)

This means that, when you spawn in a new world, your hidden saturation bar is approximately one quarter full.

I verified this claim myself by creating a new world and running /data get entity @s foodSaturationLevel, which produced 5.0f as the wiki suggested.