Count.java

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;

public class Count implements Runnable {

    private static long processedTime = 0;
    File file;

    public Count(File fileName) {
        file = fileName;
    }

    public static void main(String[] args) throws InterruptedException {

        File file = new File("/home/workspace/enwik8");
        Runnable runnable = new Count(file);
        long startTime = System.currentTimeMillis();
        Thread thread;

        for (int i = 0; i < 50; i++) {
            thread =new Thread(runnable);
            thread.start();
            thread.join();
        }

        long elapsedTime = System.currentTimeMillis() - startTime;
        System.out.println("Threads processing time " + elapsedTime + "milliseconds");
    }

    public void run() {
        int i = 0;
        int wordCount = 0;
        
        try {
            FileReader reader = new FileReader(file);
            BufferedReader buffer = new BufferedReader(reader);
            String line;
            while ((line = buffer.readLine()) != null) {
                String[] wordList = line.split(",");
                wordCount += wordList.length;
            }
            System.out.println("Number of words in " + Thread.currentThread().getName() + "-" + wordCount);

            reader.close();
        }
        catch (Exception e) {
            System.out.println(e);
        }

    }
}

I might be wrong but you are not doing any work in parallel here. thread.join(); just blocks the current thread (main thread) waiting for the completion of the other thread (50 times). You should either use service executor or create all 50 threads, call start on them and add them to the list. Then you could iterate over this list and call method join.
https://docs.oracle.com/javase/tutorial/essential/concurrency/join.html

@RostMyr is correct. This code is not executing tasks in parallel.