How to add line numbers to all lines in Google Prettify?

The root cause is list-style-type: none in prettify.css:

/* Specify class=linenums on a pre to get line numbering */
ol.linenums { margin-top: 0; margin-bottom: 0 } /* IE indents via margin-left */
li.L0,
li.L1,
li.L2,
li.L3,
li.L5,
li.L6,
li.L7,
li.L8 { list-style-type: none /* <<< THIS is the cause! */ }
/* Alternate shading for lines */
li.L1,
li.L3,
li.L5,
li.L7,
li.L9 { background: #eee }

You can either remove that rule or override it with:

.linenums li {
    list-style-type: decimal;
}