They didn’t even have horses till the Spaniards arrived in the 1500s. I find the early history fascinating, and the Indians were an amazing people, but there is so much nonsense being put out there in recent years. The sources I’ve read put the numbers no higher than several hundred thousand at best before European contact
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The American horse was wiped out along with the mega-fauna. Other than the sources you read, others have put their numbers in the several millions. Sites once considered home to a few dozen have been found by lidar to be very extensive, with populations in the thousands, even hundreds of thousands. No I do not keep links ; google, bind, duck are your new friends.
The “American” horse wasn’t something to be ridden, the entire Plains Indian culture revolving around the horse started after the introduction of the Spanish bloodlines.
I know it’s popular now to talk about millions and millions of Indians living peacefully in harmony with nature and the rest of it until the evil Whitey showed up, but it just isn’t even remotely plausible much less possible.