It's important to get representation from a variety of colleagues to create balance among the different perspectives, while constructing a clear picture of the employee’s strengths and areas to improve.
We generally recommend getting feedback from:
The line manager (supervisor)
2-3 direct reports
(You may also want to collect feedback from 2-3 customers and/or suppliers.)
Having too few feedback providers removes the anonymity from the report, and may skew the data if someone is being severely subjective.
Whereas, if you have too many feedback providers, the key messages are likely to be the same, creating unnecessary work for the people giving feedback. Also, the person receiving feedback will benefit more if feedback providers receive fewer requests, as they are likely to spend more time giving quality feedback to each person.
Of course, the right number for your specific needs might be higher or lower than our recommendation — it's up to you!