fieldOne = new FormControl('',[]);
fieldTwo = new FormControl('', []);
fieldOne.valueChanges.
.pipe(
debounceTime(100),
distinctUntilChanged(),
tap(value => {
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()