That's it. The -n can process up to n BAM files in parallel to speed up, but cannot use multiple cores on the same file.
Other HTSeq commands are muti-threaded and support the -n option though.
You could use featureCounts instead if you really need multi-threading support. Or run HTSeq count through parallel on multiple files, but this will not scale favorably complexity-wise (duplicated coordinate parsing, duplicated tree structures, increased simultaneous file IO), so this is not recommended.