CODE EXAMPLE FOR JAVASCRIPT

angular reactive input required based on previous value

// attr declarations
fieldOne = new FormControl('',[]); // initial value and validators inside array
fieldTwo = new FormControl('', []); // initialValue and validators

// ngOnInit
fieldOne.valueChanges.
.pipe(
debounceTime(100),   // dalay after every change to update your validations
distinctUntilChanged(), // only update validators when value change
tap(value => {

// depend on your options you can use switch here
   if(value === 'Adhaar number'){
    this.fieldTwo.setValidators([Validators.required,Validators.maxLength(12)]);
    this.fieldTwo.updateValueAndValidity();
  } else 
 if(value === 'PAN number'){
    this.fieldTwo.setValidators([Validators.required,Validators.maxLength(10)]);
    this.fieldTwo.updateValueAndValidity();
  } else {
this.fieldTwo.setValidators([Validators.required]);
    this.fieldTwo.updateValueAndValidity();
}
})

)
.subscribe()