埋め込んだExcelワークブックのシート数を取得するコードです。

<script type="text/javascript" src="http://r.office.microsoft.com/r/rlidExcelWLJS?v=1&kip=1"></script>
<script type="text/javascript">
    var fileToken = "SD92A165759188B352!265/-7880906317294423214/";

    if (window.attachEvent) {
        window.attachEvent("onload", loadEwaOnPageLoad);
    } else {
        window.addEventListener("DOMContentLoaded", loadEwaOnPageLoad, false);
    }

    function loadEwaOnPageLoad() {
        var props = {
            uiOptions: {
                showGridlines: false,
                showRowColumnHeaders: false,
                selectedCell: "'Sheet1'!A1"
            },
            interactivityOptions: { }
        };

        Ewa.EwaControl.loadEwaAsync(fileToken, "myExcelDiv", props, onEwaLoaded);
    }

    function onEwaLoaded(result) {
        ewa = Ewa.EwaControl.getInstances().getItem(0);
    }
    
    function fnc() {
        alert("シート数：" + ewa.getActiveWorkbook().getSheets().getCount());
    }
</script>
<input type="button" value="シート数表示" onclick="fnc();"></input><br>
<div id="myExcelDiv" style="width: 400px; height: 100px"></div>

・Ewa.SheetCollection.getCount()
http://msdn.microsoft.com/en-us/library/ee660086.aspx