As gutbag says, your character array stores 8 bit values and your ADC is set for 10 bit resolution. You will have to use a 16 bit array, signed or unsigned integer.
Also realize that your Serial,println() will cause a delay equal to the time to send the byte (will need to be two bytes) of data on the serial line. So you will miss a number of samples while this happens.
Since you are using Serial.println() there will be an extra character appended, the end-of-line character which also causes the serial buffer to flush.
If you need to examine each sample from your ADC, without missing the ones during serial transmission, you may have to store a number of samples in the array, then dump the arrary with the Serial.println in a separate loop.
A reminder, the ADC accepts signals for 0 VDC to the Analog Reference Voltage you have configured. You cannot supply it with voltage less than 0 VDC, so if you function generator is giving you AC signals you will have to bias them to fall withing 0- reference voltage.
but my "problem" now is, that I wanna read 6 analog pins one by one..... so that I get for example
5000 values from A0 , then 5000 from A1 and so on...
Did someone know how can I realize that ?
here is my code till now:
unsigned long data[5000];
void setup() {Â Â Â Â
 Serial.begin(115200);Â
 adc_init(ADC, SystemCoreClock, 21000000UL, ADC_STARTUP_FAST);
 ADC->ADC_MR |= 0x80; //set free running mode on ADC
 ADC->ADC_CHER = 0x80; //enable ADC on pin A0
 analogReadResolution(8);
}
void loop() {
 unsigned int i;
 for(i=0;i<5000;i++){
  while((ADC->ADC_ISR & 0x80)==0); // wait for conversion
  data[i]=ADC->ADC_CDR[7]; //get values
 }
 for(i=0;i<5000;i++) {
  Serial.println(data[i]);
 }
Â
delay(100);Â
}