Since you're using the JavaScript CSOM you need to fetch the properties asynchronously. Your code should look something like this:
var oList;
function theFunction() {
var sListId = SP.ListOperation.Selection.getSelectedList();
var oWeb = item_clientContext.get_web();
oList = oWeb.get_lists().getById(sListId);
// .load() tells CSOM to load the properties of this object
// multiple .load()s can be stacked
item_clientContext.load(oList);
// now start the asynchronous call and perform all commands
item_clientContext.executeQueryAsync(onSuccess, onFail);
// method will exit here and onSuccess or OnFail will be called asynchronously
}
function onSuccess(sender, args) {
alert('No of rows: ' + oList.get_itemCount());
}
function onFail(sender, args) {
alert('Request failed.\n' + args.get_message() + '\n' + args.get_stackTrace());
}
GL