var vocales = new[] { 'A', 'E', 'I', 'O', 'U' };

Console.WriteLine("Introduce un texto: ");
var text = Console.ReadLine().ToUpper();

foreach (var v in vocales)
{
    var matches = text.Count(x => x == v);
    Console.WriteLine($"Existen {matches} vocales {v}");
}