Hidden abilities are suppose to be rare , there is no specific percentage. The only way to get a guaranteed hidden ability is from events and specific areas. You can increase your chances , for example Pokémon encountered in the Friend Safari have approximately a 1 in 3 chance of having their Hidden Ability if the Friend to whom the Friend Safari corresponds has been recognized as a Friend in the Player Search System (i.e. if both players have been on the PSS at the same time).
Note from Fizz: The Friend Safari only exists in Pokemon X and Y.
Edit: Sorry I didn't realise you were talking about sword/shield
So far in Sword and Shield, the only way to get a Pokémon with Hidden Abilities is to battle and catch one in a Max Raid Battle.It won't always happen but there isn't a specfic probablity so it's based on luck.