The display of categories in the form of a pie chart quickly becomes ineligible with a larger number of categories. On one hand, the human eye can distinguish and remember a maximum of 12 categories displayed at the same time. However, the maximum number of categories that should be displayed on a pie chart is actually much smaller. For example, Excel recommends a maximum of 5-6 categories, Eagereyes recommends 5-7 categories and Haikudeck lists up to 8 categories. Most other recommendations mention 6 categories University of Leicester and Statistics Canada).
It follows from the above that it is suitable to show up to 5-7 categories on a pie chart.
It should be added, however, that pie charts (and circular forms in general) should - with few exceptions - be avoided in principle. The human cognitive and visual system is in general much better at detecting differences in lengths and heights than it is at detecting differences in areas and angles.
In particular, three-dimensional representations where thickness is added to pie charts are almost always unsuitable, as they add an unnecessary barrier to conveying key information about the relationships between categories. They often even convey the wrong information to the user.
