In the case of a large number of categories, the display of categories in a form of a pie chart can lack transparency. When we’re setting limitations, it is useful to remember that the human eye can distinguish and remember no more than 12 categories that are displayed at the same time. The suggested number of categories to use is substantially lower. Thus, for example, Excel recommends no more than 5-6 categories; Eagereyes recommends 5-7 categories; Haikudeck recommends up to 8 categories. Most of the other recommendations mention 6 categories (University of Leicester and Statistics Canada).
From the foregoing, therefore, follows that the appropriate number of categories to display in a pie chart is from 5 to 7.
It adds, however, that is better to avoid pie charts (and circle formats in general) – with few exceptions. The human cognitive and visual system is in principle much better in detecting differences between the length and height than the perception of differences within areas and corners.
We should especially emphasize that using three-dimensional presentations, where, for example, thickness is added to pie charts, is rather inappropriate. 3D presentations are an unnecessary obstacle to transmit key information about the relationships between categories to users and often send the wrong information.