How to refer to controls using variable in a loop?

You can use this using Controls.Find:

For i As Integer = 1 To 16
    Dim arrCtrl() As Control = Me.Controls.Find("label" & i, True)

    If arrCtrl.Length = 1 AndAlso TypeOf arrCtrl(0) Is Label Then
        DirectCast(arrCtrl(0), Label).Text = Math.Ceiling(Rnd() * 99)
    End If
Next

Getting a random Integer

Use the Random class instead of the Rnd function to get a random Integer within a specified range in the Random.Next(Int32, Int32) method. Declare a class variable of Random type:

Private ReadOnly rand As New Random

Finding a range of controls

This code snippet iterates over the Controls collection of the container, returns - if any - the Label controls where their names are equals to a range of names starts from label1 to label16, and finally, assign a random Integer to their Text properties:

Private Sub TheCaller()
    For Each lbl In Controls.OfType(Of Label).
        Where(Function(x) Enumerable.Range(1, 16).
        Any(Function(y) x.Name.ToLower.Equals($"label{y}")))
        lbl.Text = rand.Next(1, 100).ToString
    Next
End Sub

Just in case, if the Label controls are hosted by different containers, then you need a recursive function to get them:

Private Function GetAllControls(Of T)(container As Control) As IEnumerable(Of T)
    Dim controls = container.Controls.Cast(Of Control)

    Return controls.SelectMany(Function(x) GetAllControls(Of T)(x)).
        Concat(controls.OfType(Of T))
End Function

And call it as follows:

Private Sub TheCaller()
    For Each lbl In GetAllControls(Of Label)(Me).
        Where(Function(x) Enumerable.Range(1, 16).
        Any(Function(y) x.Name.ToLower.Equals($"label{y}")))
        lbl.Text = rand.Next(1, 100).ToString
    Next
End Sub

Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
    Dim labels As New List(Of Label) From {Label1, Label2, Label3, Label4}
    For Each l As Label In labels
        l.Text = rand.Next(99).ToString
    Next
End Sub

To use your approach

1. Declare a variable for the Random class outside your method (a Form level variable).

2. Create a List of labels.

3. Loop through all the labels in your list and set the .Text property with the .Next method of the Random class.