if (websites > 5) {
            websites = 1;
    }

<script>
    var websites = 1,
        sites = ['http://...../1.html','http://...../2.html','http://...../3.html','http://...../4.html','http://...../5.html'];
    function newSite() {
        if (websites >= sites.length) {
            websites = 0;
        }        
        document.getElementById('myIframe').src = sites[websites];
        var height=window.innerWidth;//Firefox
        if (document.body.clientHeight) {
            height = document.body.clientHeight;//IE
        }
        document.getElementById("myIframe").style.height= parseInt(height - document.getElementById("myIframe").offsetTop-8)+"px";
        websites++;
        document.getElementById("count").innerHTML="Next Pages"+websites+"/5"    
    }
</script>