In many of the LHOTP episodes, we see Charles Ingalls always wearing that white shirt with vertical red stripes. Were the Ingalls that dirt-poor, that that was the only shirt that Charles could afford to wear? Or did Charles have a closet full of those striped shirts to wear every day like the exact same clothes that Bart Simpson wore every day?

It seemed as if Caroline Ingalls and the Ingalls girls always had a number of different outfits to wear. Even their precocious adopted son, Albert, had different outfits to wear.

I guess that Charles just could not afford to buy a more extensive collection of clothes in much the same way that Charles could not afford to get a much-needed hair cut on a regular basis?

I think the mother and the girls may have liked to sew their own clothes. Charles probably had two of the same shirt and he just wasn't interested in more.

This may have followed the practice established on Bonanza where the 3 or 4 lead characters, including the one played by Landon, began wearing the same wardrobe for each in all episodes (starting in Season 2 of that series I believe). By doing that, footage from earlier episodes could be inserted as needed with the actor appearing consistently -- saving money. Usually it was distant shots of the actor riding along on a horse.

Anyway, that may not have been the LHoP situation, but perhaps Landon was used to the idea of wearing what amounted to a "uniform" and continued that practice from his Bonanza years.

^ That is a very good take, and I think you could be right.