No set number of interviews or visitors settles an assumption. What you can state is what the evidence behind it is able to establish.
A controlled comparison estimates the effect of one change on one counted action, and it says little about motive. Interviews and observed tasks show what people reported and what they did. That suits a belief about reasons, or about where a task breaks down. It doesn't estimate how a change will perform across all of a page's visitors.
So record each belief with the kind of evidence it has, and check that the evidence fits what the belief claims. A winning page test alone leaves a belief about motive unconfirmed.
Some pages won't get a comparison. How long a test needs depends on the page's traffic and on how visitors behave, as Webflow's testing documentation notes. On a page with few visitors, that can be longer than the decision can wait. The team then decides on the interviews and the observed sessions, and labels the belief with the evidence it has.
The list stays open after launch. The method's improvement cycle ends each round by learning from the data and passing the findings to marketing, sales and service. That is the point to update the assumptions. Lean Labs' growth-driven design service shows where new evidence comes from: it draws on page behavior, form and conversion behavior, CRM lifecycle stages and sales feedback. Any of those can contradict a belief that looked sound at launch.
When a belief fails, trace it back to the plan. Look again at the priorities that depended on it before you commit the next cycle's work.