That number varies depending on who you ask, with CR being one of the sources that claims a heavier share of dub watchers, but half their business is now dubbing/localization (by way of Funimation merger) so it may not be the most objective source.

Either way it’s bad business to throw some of your most loyal customers under the bus, regardless of how big of a portion they represent. Dub viewers tend to be more casual and fickle and will largely evaporate once the zeitgeist of popular media moves from anime onto something else.