private void button2_Click(object sender, EventArgs e)
{
    int number = int.Parse(textBox3.Text);
    Button[] textBoxes = new Button[number];
    int location = 136;

    for (int i = 0; i < textBoxes.Length; i++)
    {
        location += 81;
        var txt = new Button();
        textBoxes[i] = txt;
        txt.Name = "text" + i.ToString();
        txt.Text = "textBox" + i.ToString();
        txt.Location = new Point(location, 124);
        txt.Visible = true;
        this.Controls.Add(txt);
    }
}

class Form1 : Form {
    private Button[] _textBoxes;

    private void button2_Click(object sender, EventArgs e) {
        int number = int.Parse(textBox3.Text);
        if(_textBoxes != null) {
            foreach(Button b in _textBoxes)
                this.Controls.Remove(b);
        }

        _textBoxes = new Button[number];
        int location = 136;

        for (int i = 0; i < textBoxes.Length; i++) {
            location += 81;
            var txt = new Button();
            _textBoxes[i] = txt;
            txt.Name = "text" + i.ToString();
            txt.Text = "textBox" + i.ToString();
            txt.Location = new Point(location, 124);
            txt.Visible = true;
            this.Controls.Add(txt);
        }
    }
}