Вам нужно
let label1 = UILabel()
label1.text = "123"
view.addSubview(label1)
label1.translatesAutoresizingMaskIntoConstraints = false
let label2 = UILabel()
label2.text = "456"
view.addSubview(label2)
label2.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint.activate([
label1.topAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.topAnchor,constant:20.0),
label1.centerXAnchor.constraint(equalTo:self.view.centerXAnchor),
label2.topAnchor.constraint(equalTo: self.label1.bottomAnchor,constant:20.0),
label2.centerXAnchor.constraint(equalTo:self.view.centerXAnchor)
])
Вы также можете использовать вертикальный UIStackView