There is no fixed number of pages that makes you an authority, and any specific count you have seen quoted is invented, because authority is not earned by reaching a page total at all. It is earned by covering a topic’s real sub-questions completely, then getting that coverage to actually rank and be cited. The honest answer reframes the question entirely: the thing you are really asking is not how many pages, but whether you have covered the topic thoroughly enough that a reader, and a search engine, treat you as the place that answers it. Page count is a side effect of doing that, not a target you hit.
The reason no number exists is that topics differ enormously in scope. A narrow, well-bounded subject may have only a handful of genuine sub-questions, and covering all of them well could make you authoritative with very few pages. A sprawling subject may have hundreds of distinct questions, and covering a fraction of them, however many pages that takes, would leave you looking incomplete. So the same count means authority in one topic and thinness in another. A number stripped of its topic is meaningless, which is why anyone offering one is guessing.
It is worth keeping the uncertainty visible rather than papering over it. Authority is also not something you can declare from your side. It is conferred: it shows up when your pages rank for the topic’s queries, when other sites and writers cite you, and when readers return to you as a reference. Those are outcomes you influence but do not control on a schedule, so the timeline and the exact effort are genuinely uncertain, and that is the truthful state of things, not a gap to fill with a made-up figure.
What you can control is completeness of coverage. Map the real questions people ask about the topic, the obvious ones and the long-tail edges, and answer each one well enough that there is no gap a competitor obviously fills better. When the coverage is complete and the individual answers are strong, the page count takes care of itself and the recognition follows.
So instead of counting pages toward an imagined threshold, sit down and map the topic’s actual sub-questions, then build until each one is genuinely covered. Measure your progress by gaps closed, not by pages published, and let authority be the result of complete coverage rather than a number you chased.