The random() function can be used to generate pseudo-random numbers:

void setup() {
    Serial.begin(9600);
}

void loop() {
    long randomNumber = random(500);  // Generate a random number between 0 and 499
    Serial.println(randomNumber);

    randomNumber = random(100, 1000); // Generate a random number between 100 and 999
    Serial.println(randomNumber);

    delay(100);
}

If it is important for a sequence of numbers generated by random() to differ, it is a good idea to specify a seed with randomSeed():

void setup() {
    Serial.begin(9600);
    
    // If analog pin 0 is left unconnected, analogRead(0) will produce a
    // different random number each time the sketch is run.
    randomSeed(analogRead(0));
}

void loop() {
    long randomNumber = random(500); // Generate a random number between 0 and 499
    Serial.println(randomNumber);

    delay(100);
}