int sensor = 900;                 //variabel sensor

void setup() {
  Serial.begin(9600);
  while (!Serial);
  randomSeed(analogRead(A0));
}

int number() {
  int a = random(0, 10);          //range pengurang nilai sensor
  int d = random(500, 1500); //range delay perubahan nilai
  if (sensor < 670) {
    sensor = 900;
  }
  sensor -= a;
  delay(d);
  return sensor;
}

void loop() {
  int val = number();
  Serial.println(val);
}

I modified the delay for test.

randomSeed ensures different random values at every run

int sensor = 900;                 //variabel sensor

void setup() {
  Serial.begin(9600);
  while (!Serial);
  randomSeed(analogRead(A0));
}

int number() {
  int a = random(0, 10);          //range pengurang nilai sensor
  int d = random(500, 1500); //range delay perubahan nilai
  if (sensor < 670) {
    sensor = 900;
  }
  sensor -= a;
  delay(d);
  return sensor;
}

void loop() {
  int val = number();
  Serial.println(val);
}

I modified the delay for test.

randomSeed ensures different random values at every run

EDIT: a not blocking version which returns the same values for random interval:

int sensor = 900;                 //variabel sensor
unsigned long nextChangeMillis = 0;

void setup() {
  Serial.begin(9600);
  while (!Serial);
  randomSeed(analogRead(A0));
}

int number() {

  if (millis() < nextChangeMillis)
    return sensor;

  int a = random(0, 10);          //range pengurang nilai sensor
  int d = random(500, 1500); //range delay perubahan nilai
  if (sensor < 670) {
    sensor = 900;
  }
  sensor -= a;
  nextChangeMillis += d;
  return sensor;
}

void loop() {
  int val = number();
  Serial.println(val);
}