The ratings of National Public Radio have been doing fairly well in recent years as the huge Baby Boomer audience loses interest in music as they age. NPR likes to publish their audience demographics to attract advertisers or sponsors or whatever they call them (e.g., "NPR listeners are extraordinarily well-educated.") What NPR won't tell us in anywhere I've been able to find online is how few NPR listeners are Hispanic / Latino. This factsheet from NPR implies that 14% of NPR listeners aren't white, with the largest minority group being blacks at 5% (blacks make up 31% of the audience for NPR jazz shows, which is nice to hear).
So, my best guess is that the answer to the title question is: Not many.
NPR is kind of a business, kind of an identity politics totem dedicated to making America more NPRish. NPR claims to have 28 million listeners, so how many more will the next 50 million Hispanics add? Maybe a million? Now the way I tend to look at things, adding 50 million people to the country over the next couple of decades and adding only 1 million NPR listeners makes the country less NPRish. But nobody at NPR seems to see it that way. Am I missing something?