Q:

How to count the total number of words from all the files in a directory?

It could be with anything: terminal, Vim, Atom. How to count the total number of words from all the files in a directory?

A:

If you just want the total number of words in all the files in a directory (and assuming no sub directories, ignoring hidden files and other caveats), you could try:
cat * | wc -w

cat * simply concatenates the content of all the files in the current directory to standard output. This is piped into wc -w wc (word count) simply returns the number of words that (in this case) it reads from standard input.
If you're ok with slightly more complex output, simply try:
wc -w *

wc has other options that return number or lines, number of characters etc.

