If you include HoH people, people who can't access audio at the moment, people who don't want to use audio at the moment (maybe have some music going on) etc. The number of people whose lives will have been improved through implementing usability for Deaf/deaf people will probably be a significant portion of the population.

I'm not HoH/deaf/Deaf but I know I'd definitely love the opportunity to have appropriate captions during calls because I sometimes struggle to pick up words when they're spoken quickly to me, especially on highly technical or complex subject matter as required for my job.