I have found the answer to my question after much searching and doing some development.
First you have to build an iframe to the /_layouts/15/sitesubs.aspx page. Then you need to write a javascript to call the iframe's id and the id of the specific "select" function id. (Code sample below)
Thanks for looking. Also, please note, only those who have admin access will be able to view the results so I have added an additional if statement to clear out the div from view if the user cannot view the sitesubs page.
<style>
#iframe1
{
width: 200px;
height: 200px;
overflow: scroll;
display: none;
}
#iframeCountDisplay
{
margin-top: 2px;
margin-bottom: 6px;
background-color: #B0E2FF;
}
</style>
<iframe
src="https://URL/SiteCollection/SiteName/_layouts/15/sitesubs.aspx"
id="iframe1"
name="iframe1">
</iframe>
<div id="iframeCountDisplay" style="width: 45%; color: gray;">Checking Permissions...</div>
<script type="text/javascript" language="javascript">
setTimeout(function()
{
if (document.getElementById('iframe1').contentDocument.getElementById('ctl00_PlaceHolderMain_DdlUserName'))
{
document.getElementById('iframeCountDisplay').innerHTML = "Loading...";
var subCount = "Subscriber Count:"
var howMany = document.getElementById('iframe1').contentDocument.getElementById('ctl00_PlaceHolderMain_DdlUserName').length -1;
document.getElementById('iframeCountDisplay').style.width = "41%";
document.getElementById('iframeCountDisplay').style.color = "black";
document.getElementById('iframeCountDisplay').innerHTML = subCount + " " + howMany;
}
if (document.getElementById('iframe1').contentDocument.getElementById('ms-error-body'))
{
document.getElementById('iframeCountDisplay').style.display = "none";
}
}, 2000);
</script>
