Thousands of individual QR codes

The number of codes is not limited. But be aware that standard qr codes are generated on your server. So if you use shortcodes to generate them dynamically your load increases. For frequently called pages you are better off storing the static image instead.

What is the php code to insert it directly in a page ?

You insert it with shortcuts. A description of shortcuts can be found in the plugin (settings->QR Generator 4 All) and looks like this:

[qrgen4all data=”Hello World!”]
[qrgen4all data=”mailto:[email protected]?subject=Your subject” size=”200″ margin=”4″ fg=”222222″ bg=”ffffa0″ class=”myfloating” style=”float:right;margin:10px” alt=”Mail me” title=”Mail me”]