Assuming you are using Linux or OS X, the easiest way is probably to use wp-cli (if present in your WordPress installation) to return a list of all posts:
wp-cli post list
Then pipe it to the word count tool to get the number of lines:
wc -l
Finally, deduct one to take care of the header line which is not a post:
awk '{print $1-1}'
So, in one line:
wp-cli post list | wc -l | awk '{print $1-1}'