As you can see in gatsby-ssr-js docs, onRenderBody exposes a bunch of props where there's a pathname.

pathname {string}

The pathname of the page currently being rendered.

That said, you can try something like:

const React = require("react")

exports.onRenderBody = ({ setPostBodyComponents, pathname }) => {
   const SCRIPT_PAGES = ['/page-1', '/page-2'];

   if(SCRIPT_PAGES.includes(pathname)){
      setPostBodyComponents([
        <script dangerouslySetInnerHTML={{ __html:`your script here`}} />,
    ]);
  }
};

In that case, your SCRIPT_PAGES will hold the pages where the script will be inserted. Using that approach, you can tweak it as you want.