you've discovered the lie of "separate but equal". Sure, not that many buildings had redundant toilets. Instead, black people simply didn't have one they could use. It was very common for buildings to have only a "White's only" restroom.
I simply don't know what to tell you if you don't believe that racial segregation in public bathrooms was not rigorously enforced. This is a stunning ignorance of the relevant history.
You are correct that there were not redundant restrooms everywhere. Black people simply did not have access to public restrooms in white parts of society. This was a small facet of Jim Crow that intended to keep blacks in their place, by making simple existence outside the designated niche established by the white supremacist state impossible. Outside of the black parts of town, it would be very difficult indeed to perform the basic biological necessities of life, be it using toilets, buying food, using transit, and so on. That was the whole point, to prevent comingling of blacks and whites by building these barriers that made sharing the same spaces impractical.
Finding a place to take a piss was such a problem for black motorists that a popular guide book, the Green Book, was published listing restrooms, hotels, and other necessities that were available to black patrons.
https://en.wikipedia.org/wiki/The_Negro_Motorist_Green_Book
