What is the maximum values that can be added in a custom segment?
You should be able to create unlimited values on Custom Segment as the Custom Segment feature mimics the Department and Class as an additional classification to categorize your records however you may experience slower performance when having a huge amount of values on the segment.
You may also refer to these SuiteAnswers articles for more information:
ID:22873 – Limitation to the Number of Classes and Departments that a User may Create
ID:43329 – Benefits of Custom Segments