If you have a large website then chances are that you will analytics data to give you an idea if there is a high dropout rate due to usability issues in the form of bounce rate statistics and other quantitative data.
Depending if you have configured the analytics and tracking properly, you will be able to pick up the conversion or task completion rate for specific activities. You can then compare this to the number of complaints that people make to get an approximate idea of the % of people that complain about a specific problem/issue.
However, you cannot necessarily extrapolate this to same type of issues or even other types of problems because the impact to the users is not necessarily related to the severity of the issue.
But over time you should be able to establish some baseline for the user sentiment if you can collect data from different touch points.
Also, if you show that you are willing to take action based on user feedback and complaints, you are also likely to increase the rate of user reporting issues over the lifetime of your product or service.