From the phrasing of your question it sounds like you are looking for a quantitative solution ("what percentage of the 100 pages" and "constrain them to five answers"). It is really hard to design surveys properly. You need to ask the right questions to the right people. You need to be aware of self-selection of participants, and how this may impact your site's target demographic. There are lots of issues, and books have been written about this topic.
What I would suggest, as a start, is to not worry about statistics too much. Rather opt for a more qualitative approach, and ask people two simple questions:
The first question will allow people to list things they like, in any order they like. If you aggregate all the results, it should (hopefully) align with your analytics data.
The second question will do three things:
Show you what people think they want from your site, but limit them to be very specific. If they could only list one thing, they will most likely list the most important thing to them. This is valuable information.
Highlight which portions of the site are actually useful, but there are people who do not know that it exist or where to find it (assuming they name something that already forms part of your site)
Provide you with a starting point from which to prioritize your redevelopment efforts.
Maybe people don't have a problem with your site. Maybe they actually want a better mobile version or app (as a hypothetical example). If you ask 100 people, and five say they want a mobile app, is it enough? That is a decision for your management team. But still, you would not have known about this apparent need in a structured "list your top 20 pages" style questionnaire format.