From Xojo Documentation

Property (As Double )
aiOSProgressBar.MaxValue = newDoubleValue
DoubleValue = aiOSProgressBar.MaxValue

Supported on Mobile (iOS).

The maximum value of the progress bar. When CurrentValue reaches MaxValue, the progress bar appears "full".

Sample Code

Only update the current value if it is below the maximum:

If MyProgressBar.Value < MyProgressBar.MaxValue Then
MyProgressBar.Value = MyProgressBar.Value + 1
End If