In one of my previous blog I explained,how a rollup field can be used to count child records : Count the number of related child records using a Rollup field

However, Rollup field is calculated with an Async System Job, therefore, if you need something to be triggered on every form load; you will have to write a JS.

Here is a quick code to count the number of related sub-grid record on a form onload. You can put an alert of the count or add that number to a field.

function getTotalGridRecordCount() {
 debugger;
 try {

setTimeout(function () {
 if (Xrm.Page != null && Xrm.Page != undefined && Xrm.Page.getControl("contactopportunitiesgrid") != null && Xrm.Page.getControl("contactopportunitiesgrid") != undefined) {
 var count = Xrm.Page.getControl("contactopportunitiesgrid").getGrid().getTotalRecordCount();
 alert("Total Opportunities:"+count);

}
 }, 5000);
 }
 catch (e) {
 Xrm.Utility.alertDialog(functionName + "Error: " + e.message || e.description);
 }
}

Add script to the form and call this function on Onload event. Open the form to see it in action:

Hope this helps! cheers!