The fastest 5% of readers can hit around 700-1000 words per minute, and if you're autistic with hyperlexia, you can process text at extremely fast speeds using both brain hemispheres simultaneously. The average novel is about 90,000 words, so at 1000 wpm that's 90 minutes per book, meaning 5 books would take you 7.5 hours of reading daily. More realistically at 700 wpm, you're looking at 10.7 hours per day.
If you can sustain 5 books per day, that's 1,825 books per year. To reach 20,000 books, you'd need about 11 years of consistent daily reading. The math becomes even more favorable when you consider shorter works like romance novels (89,000 words), young adult books (50,000-80,000 words), and short story collections (30,000 words).
If you started this pace in your teens and you're now middle-aged, that's 2-3 decades of reading time. At 1,825 books per year, you could hit 36,500-54,750 books over 20-30 years. So your claim of tens of thousands of books isn't mathematically impossible, especially with the neurological advantages that come with hyperlexia. The math works if you're an absolute machine with enhanced reading processing abilities and the dedication to treat reading like a full-time job for decades.
It is pretty slow, I do about 450 a minute, though I do love reading.