For example, we can assume that the text file is comma separated and the field being counted is the second field.
cut -d ',' -f 2 /files/data.csv | uniq | wc -l
On the other hand, if you also wanted a count of how many times each value has occurred, then use the following command:
cut -d ',' -f 2 /files/data.csv | uniq -c
Simple does it - no need for awk or sed!