Mihkel, an Objective Digital partner from Realeyes.it (love that URL!) in the UK, has created this compelling example that shows you need 35-50 people to draw valid conclusions from eye tracking. However, he holds that useful information can be gained from fewer participants.
From my perspective, tools that you use to help in usabilty all give you a different perspective on the technlogy you are working on. Whether it be eye tracking, personas, tasks, usability, surveys, the list goes on.
It’s all about choosing the right one for your context.