//Erstellen der Textboxen
for (int Index = 1; Index ≤TextboxAnz; Index++)          
{              
    TextBox pTextbox = new TextBox();              
    pTextbox.ID = "TextBox" + Index;              
    pTextbox.Text = "Bitte Text eingeben"; 

    Placeholder_Textbox.Add(pTextbox); 
}

//Abfrage nach ButtonEvent
string[] pResultString = new string[TextboxAnz];

for (int Index = 1; Index ≤ TextboxAnz; Index++)
{
    string pTextBoxID = "TextBox" + Index;
    TextBox pTextbox = (TextBox)Placeholder_Textbox..FindControl(pTextBoxID);

    if (pTextbox != null)
    {
        pResultString[Index-1] = pTextbox.Text;
    }
}

foreach(Control cntrl in Controls) {
lblTest.Text += cntrl.ID + "<br>";
}