7 segment display + button

Using Arduino Displays
4

Try this for the Loop() portion of your code:

void loop() {

  delay(500);
  if (digitalRead(drukknop)) {
    knopteller++;
    if (knopteller > 9)
      knopteller = 0;
    Serial.println(knopteller);
  }

  switch (knopteller) {
    case 0: zero(); break;
    case 1: one(); break;
    case 2: two(); break;
    case 3: three(); break;
    case 4: four(); break;
    case 5: five(); break;
    case 6: six(); break;
    case 7: seven(); break;
    case 8: eight(); break;
    case 9: nine(); break;
    case 10: zero(); break;
  }
}

This assumes that pin 9 is HIGH to increment the counter. Two times per second the counter will increment when the button is pushed. Crude but it shows the logic required to detect the pushbutton on pin 9.

You do not need to test if a boolean variable is high like this:

if (pin == HIGH) { // true when pin == 1 }

this can be shortened to:

if (pin) { // true when pin == 1 }

If the test is for a LOW input:

if (!pin) { // true when pin == 0 }