When I was in radio, I believe the rule was 30 minutes per week of public-service-oriented content. Of course, that was over 10 years ago; it might have changed.
Stations had a choice of how they put the 30-minutes together. They could run a 30-minute public affairs show, or they could air 30 individual minutes. They had to document it for their public file, though, so every station I ever worked at used the 30-minute show format, in order to have some "meat" to throw in the public file.
WIOT and WCWA used to run a show on Sunday mornings at like 5AM called "Question This." That was their Public affairs fulfillment show. I am not sure if it is still on the air or not. I think 93.5 and 94.5 run a similar show, with London Mitchell?
And yes, too many PSAs usually means too few ads sold. Occasionally, you'll hear a PSA used as filler, when a single spot wasn't sold. Sometimes you'll hear one thrown in to fill time during a moment of "technical difficulties," like during a sports event. But if you hear PSAs back-to-back-to-back during a standard commercial break...yeah, that's trouble.
The PSAs used to come on reel-to-reel tapes or CDs, and they would have multiple cuts for various uses. Different lengths, sometimes one in Spanish, and usually ones with different music beds and voice-overs meant for different formats, i.e. country, R&B, A/C, etc...
One of my jobs up in Michigan was to program PSAs as filler during talk shows on our "flanker" AM station; the up-north equivalent of 1230 and 1470. Management never listened to that little station, so I used to have fun programming the Spanish-language PSAs during the G. Gordon Liddy show...