On Mac OSX, the default localhost folder is located at /Library/WebServer/Documents

P.S Tested with MacOS Sierra, 10.12.6

Find httpd.conf

By default, Apache is enabled and installed in /etc/apache2/, inside httpd.conf file, find DocumentRoot to tell where is the default localhost folder.

sudo vim /etc/apache2/httpd.conf

# DocumentRoot: The directory out of which you will serve your
# documents. By default, all requests are taken from this directory, but
# symbolic links and aliases may be used to point to other locations.
#
DocumentRoot "/Library/WebServer/Documents"
<Directory "/Library/WebServer/Documents">
    #...
</Directory>

References

  1. Apache – Mapping URLs to Filesystem Locations