In fact many of them don’t, since the body is mostly symmetrical and apart from cutting them open or doing an MRI, you can’t really tell (which isn’t a big deal in most cases, because most medical procedures work regardless of this condition). Also, the heart is located almost in the middle, so there is not much difference.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8901252/
https://www.healthline.com/health/situs-inversus#symptoms
https://my.clevelandclinic.org/health/diseases/23486-situs-inversus
Of course, trying to estimate how many people don’t know about a disease is a difficult task, but the general consensus is the condition is rare and often doesn’t produce any symptoms, as such there are definitely many people with the condition that haven’t even ever heard of it.