Looking back at my last re-read, I'd say the suspenseful build really kicks into high gear around Chapter 20. The book has 39 chapters total, but it's not just a simple count from a certain point. Before that, you get the whole setup with the gates and the keys, which has its own rising tension, but Chapter 20 is where the corporate stakes become immediate and the lead feels genuinely threatened. From there, it's this relentless sprint with the IOI hunt and the final gate puzzles that doesn't let up until the last page.
Some people point to Chapter 18, right after the second gate, as the turning point. I see their logic—that's when the protagonist's real-world situation gets dangerous. But for pure, page-turning 'what happens next' dread mixed with puzzle-solving urgency, the stretch from 20 through 38 is masterfully constructed. The chapters get shorter, the pacing becomes almost breathless, and every new obstacle feels like it could be the one that ends everything. It's less about a specific number and more about how Cline structures that final act to mirror a video game's final level, with each chapter acting like a sub-boss or a tricky platforming section before the big bad.
The epilogue (Chapter 39) is pure resolution, so all the suspense is paid off by then. What's clever is how the 'suspense' isn't just about winning the contest; it's woven with worries about his friends, his identity, and the fate of the OASIS itself. So those later chapters are doing triple duty on the tension front.