How to add Facebook pixel code to my website
Craft makes no assumptions on how your site should be built so it's impossible for us to say exactly where to put the pixel code. 

You would add the code to whichever template you needed it in. This depends entirely on how your site is put together.

Oftentimes the &lt;head&gt; portion will be within your main _layout file but that is not necessarily always the case. This file can also be anywhere within your templates directory.

For Craft 2, look in your /craft/templates directory
For Craft 3, look in your /templates directory