Here is The Code,

function validateSubgridCount(executionContext) {
    // Get the form context
    var formContext = executionContext.getFormContext();

    // Get the lookup field value
    var lookupField = formContext.getAttribute(“vijay_participants”); // Replace with your lookup field name
    if (!lookupField || !lookupField.getValue()) return;

    // Retrieve the count field value from the lookup record
    var lookupId = lookupField.getValue()[0].id.replace(“{“, “”).replace(“}”, “”);
    Xrm.WebApi.retrieveRecord(“vijay_participants”, lookupId, “?$select=vijay_count”).then(
        function(result) {
            var lookupCount = result.vijay_count; // Correct field name for count

            // Prepare FetchXML to get related records in the subgrid
            var fetchXml = `<fetch aggregate=”true”>
<entity name=”vijay_hotel”> <!– Replace with your subgrid entity name –>
<attribute name=”vijay_hotelid” alias=”recordCount” aggregate=”count” />
<filter>
<condition attribute=”vijay_participants” operator=”eq” value=”${lookupId}” /> <!– Dynamic lookup ID –>
</filter>
</entity>
</fetch>`;

            // Execute the FetchXML query
            Xrm.WebApi.retrieveMultipleRecords(“vijay_hotel”, “?fetchXml=” + encodeURIComponent(fetchXml)).then(
                function(response) {
                    var subgridRecordCount = response.entities.length > 0 ? response.entities[0].recordCount : 0;

                    // Check if subgrid count exceeds the lookup count
                    if (subgridRecordCount > lookupCount) {
                        // Show error if count is exceeded
                        formContext.ui.setFormNotification(
                            “The number of records in the subgrid exceeds the allowed count in the lookup record.”,
                            “ERROR”,
                            “count_exceeded”
                        );
                    } else {
                        // Clear notification if within limit
                        formContext.ui.clearFormNotification(“count_exceeded”);
                    }
                },
                function(error) {
                    console.log(“Error retrieving subgrid records: ” + error.message);
                }
            );
        },
        function(error) {
            console.log(“Error retrieving lookup record: ” + error.message);
        }
    );
}

Now Save And Publish .