QScrollArea *ScrollArea=new QScrollArea;
QVBoxLayout *VLayout=new QVBoxLayout;
QWidget *widget_in_ScrollAra=new QWidget;

widget_in_ScrollAra->setLayout(VLayout);

for(int i=0;i<20;i++){
     QLabel *Label=new QLabel;
     Label->setText(QString::number(i));
     VLayout->setWidget(Label);
}
ScrollArea->setWidget(widget_in_ScrollAra);

That's how I'm setting my ScrollArea, But is there a way to add those Label into ScrollArea by making Labels children of widget_in_ScrollAra without using any Layout?

You could use /override
http://doc.qt.io/qt-5/qwidget.html#showEvent
to first call it when shown.

Alternatively, you must know size of scroll area and calculate how many is shown using
buttons height. Since its not visible yet, it is not scrolled so should be pretty forward.

Can i ask WHY you need to know how many is visible ?

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);

      area=new QScrollArea(this);
      area->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOn);
    for(int i=0;i<23;i++){
        button=new QPushButton;
       button->setText(QString::number(i));
        layOut->addWidget(button);
    }
setlabels();

setlabels() function:

 P=area->findChildren<QPushButton*>();
    for(int i=0;i<P.count() ;i++)
{
        if(!P.at(i)->visibleRegion().isEmpty())
      {
                visibleButtonVectorBottom.push_back(P.at(i));
     }
 }
   

For me it counted the visible lables for me.
With or without scrollbar. with/without scrolling.

but they must be visible on screen before u count.

 // count visible
  int count = 0;
  QList<QLabel*> labels = ui->scrollArea->findChildren<QLabel*>();
  for ( QLabel* label : labels) {
    if ( ! label->visibleRegion().isEmpty() ) { count++; }
  }

  qDebug() << count;

Note that if last label is only partly visible, its still counted.

There's scrollbar in my scenario and I've put a few labels in it.
Now i have a slot which gets called when I scroll, and it works fine.
That slot gives the visibleRegion of the QScrollArea.
In constructor I have put 20 Qlabels in the scroll area so when the code gets executed the scrollbar allready has 20 Labels.
on that time i want to know how many labels are visible in the scrollArea.