You are using the "Feil" variable to try to count the number of failures, correct?
But it will always be at most one, because it's declared a local variable and thus won't "survive" for subsequent clicks. You check ifi the guess is wrong,. then increment feil if it is, but feil will always be zero at that point, so the increment will only ever make it 1.
Move the declaration so that it is a member variable- remove the int Feil=0 inside the button click and move it to above the click event as
private int feil=0
Then it will count to 7... And beyond, but I suppose that will be your next problem to tackle