How to count unique values from predefined range of rows from a 3rd column in a file

I have tsv file containing 8 column and 10000 rows. I need to take unique count of values at every 1000 rows interval from column 7.
Please suggest the linux commands to get desired output. I tried following command but it did not work:

awk >myfile 'NR>=1&&NR<=1000' | sort | uniq -c