Any guesses - what percentage of authors and actors use pseudonyms or stage names?

I think very common among actors and actresses and less common among authors these days. Female authors often went under male pseudonyms back in the olden days as they would get more exposure, respect and reception under a male pen name.

I agree. ^^^ I would put current time authors at 15%, actors at about half.
Actor pseudos has dropped off since the seventies, I think.
It was quite popular to have one after the communism hearings, because actors were blacklisted, so they worked under other mames. Eventually the practice began to drop off.

A lot of the pressure for actors to change their names was from the studio system, which waned actors and actresses to sound “all-American, i.e., white Anglo Saxon Protestant. Not Italian, or Jewish, or German. Spanish only if you played a Latin lover.

I’m guessing for actors it’s 50%.