How to count number of values in a dimension for a segment

[dleverentzPT]

I'm trying to show the number of unique values/rows in a dimension. Specifically, we collect the UserID of each visitor. I want to show the number of unique UserIDs within a segment. 

I tried using the Summary Number visualization, but that sums the values of the occurrences. 

[Jennifer_Dungan]

Create a calculate metric using "Approximate Distinct Count" function.

If you want to remove the "unspecified" values. you can also add an on-the-fly segment for "UserId exists" (simply drag your UserId dimension into the calculated metric above the "Approximate Count Distinct" function

[Alexis_Cazes_]

For more details on count distinct read this article https://www.fullstackanalyst.io/blog/adobe-analytics/cool-approximate-count-distinct-use-cases-adobe... it will complement the answer above