You could override report.minimal_layout to introduce a new class which blocks / hides when the total pages are greater than 1.
<template id="report.minimal_layout">
			<t t-raw="'<base href=%s>' % base_url"/>
		    <!DOCTYPE html>
		    <html style="height: 0;">
		        <head>
		            <link href="/report/static/src/css/reset.min.css" rel="stylesheet"/>
		            <link href="/web/static/lib/bootstrap/css/bootstrap.css" rel="stylesheet"/>
		            <link href="/website/static/src/css/website.css" rel="stylesheet"/>
		            <link href="/web/static/lib/fontawesome/css/font-awesome.css" rel="stylesheet"/>
		            <style type='text/css'><t t-raw="css"/></style>
		            <t t-if="subst is True">
		                <script>
		                    function subst() {
		                        var vars = {};
		                        var x = document.location.search.substring(1).split('&');
		                        for (var i in x) {
		                            var z = x[i].split('=', 2);
		                            vars[z[0]] = unescape(z[1]);
		                        }
		                        var x=['frompage', 'topage', 'page', 'webpage', 'section', 'subsection', 'subsubsection'];
		                        for (var i in x) {
		                            var y = document.getElementsByClassName(x[i]);
		                            for (var j=0; j<y.length; ++j)
		                                y[j].textContent = vars[x[i]];
		                        }
		                        
		                        var operations = {
							        'single-page': function (elt)
							        {
							            elt.style.display = (vars.topage >1) ? "block" : "none";
							        },
							    };
										
							    for (var klass in operations)
							    {
							        var y = document.getElementsByClassName(klass);
							        for (var j=0; j<y.length; ++j)
							            operations[klass](y[j]);
							            }
		                    }
		                </script>
		            </t>
		        </head>
		        <body class="container" onload="subst()">
		            <t t-raw="body"/>
		        </body>
	    	</html>
		</template>
Then, you can override the external_layout_footer as well:
<ul class="list-inline single-page">
		                <li>Page:</li>
		                <li><span class="page"/></li>
		                <li>/</li>
		                <li><span class="topage"/></li>
		            </ul>
Now, since we included the single-page class, that class will only be shown if the topage value is greater than 1.
Let me know if you need more help on this.