user29266: How many Ultrasonic sensors (HC SR04 ) can be connected to an Arduino Uno R3 board?

user6826: If you look into any example sketch, like this from sparkfun:
/**
 * HC-SR04 Demo
 * Demonstration of the HC-SR04 Ultrasonic Sensor
 * Date: August 3, 2016
 * 
 * Description:
 *  Connect the ultrasonic sensor to the Arduino as per the
 *  hardware connections below. Run the sketch and open a serial
 *  monitor. The distance read from the sensor will be displayed
 *  in centimeters and inches.
 * 
 * Hardware Connections:
 *  Arduino | HC-SR04 
 *  -------------------
 *    5V    |   VCC     
 *    7     |   Trig     
 *    8     |   Echo     
 *    GND   |   GND
 *  
 * License:
 *  Public Domain
 */

// Pins
const int TRIG_PIN = 7;
const int ECHO_PIN = 8;

// Anything over 400 cm (23200 us pulse) is "out of range"
const unsigned int MAX_DIST = 23200;

void setup() {

  // The Trigger pin will tell the sensor to range find
  pinMode(TRIG_PIN, OUTPUT);
  digitalWrite(TRIG_PIN, LOW);

  // We'll use the serial monitor to view the sensor output
  Serial.begin(9600);
}

void loop() {

  unsigned long t1;
  unsigned long t2;
  unsigned long pulse_width;
  float cm;
  float inches;

  // Hold the trigger pin high for at least 10 us
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // Wait for pulse on echo pin
  while ( digitalRead(ECHO_PIN) == 0 );

  // Measure how long the echo pin was held high (pulse width)
  // Note: the micros() counter will overflow after ~70 min
  t1 = micros();
  while ( digitalRead(ECHO_PIN) == 1);
  t2 = micros();
  pulse_width = t2 - t1;

  // Calculate distance in centimeters and inches. The constants
  // are found in the datasheet, and calculated from the assumed speed 
  //of sound in air at sea level (~340 m/s).
  cm = pulse_width / 58.0;
  inches = pulse_width / 148.0;

  // Print out results
  if ( pulse_width > MAX_DIST ) {
    Serial.println("Out of range");
  } else {
    Serial.print(cm);
    Serial.print(" cm \t");
    Serial.print(inches);
    Serial.println(" in");
  }

  // Wait at least 60ms before next measurement
  delay(60);
}

You can see it is used only two pins. So, as long as you can provide power to sensors and pins to it, you can have as much as you like. You can even expand with more IO pins using other electronics or go into some kind of multiplexing.

29266: <p>How many Ultrasonic sensors (HC SR04 ) can be connected to an Arduino Uno R3 board?</p>
 6826: <p>If you look into any example sketch, like this from sparkfun:</p>

<pre class="lang-c prettyprint-override"><code>/**
 * HC-SR04 Demo
 * Demonstration of the HC-SR04 Ultrasonic Sensor
 * Date: August 3, 2016
 * 
 * Description:
 *  Connect the ultrasonic sensor to the Arduino as per the
 *  hardware connections below. Run the sketch and open a serial
 *  monitor. The distance read from the sensor will be displayed
 *  in centimeters and inches.
 * 
 * Hardware Connections:
 *  Arduino | HC-SR04 
 *  -------------------
 *    5V    |   VCC     
 *    7     |   Trig     
 *    8     |   Echo     
 *    GND   |   GND
 *  
 * License:
 *  Public Domain
 */

// Pins
const int TRIG_PIN = 7;
const int ECHO_PIN = 8;

// Anything over 400 cm (23200 us pulse) is "out of range"
const unsigned int MAX_DIST = 23200;

void setup() {

  // The Trigger pin will tell the sensor to range find
  pinMode(TRIG_PIN, OUTPUT);
  digitalWrite(TRIG_PIN, LOW);

  // We'll use the serial monitor to view the sensor output
  Serial.begin(9600);
}

void loop() {

  unsigned long t1;
  unsigned long t2;
  unsigned long pulse_width;
  float cm;
  float inches;

  // Hold the trigger pin high for at least 10 us
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // Wait for pulse on echo pin
  while ( digitalRead(ECHO_PIN) == 0 );

  // Measure how long the echo pin was held high (pulse width)
  // Note: the micros() counter will overflow after ~70 min
  t1 = micros();
  while ( digitalRead(ECHO_PIN) == 1);
  t2 = micros();
  pulse_width = t2 - t1;

  // Calculate distance in centimeters and inches. The constants
  // are found in the datasheet, and calculated from the assumed speed 
  //of sound in air at sea level (~340 m/s).
  cm = pulse_width / 58.0;
  inches = pulse_width / 148.0;

  // Print out results
  if ( pulse_width &gt; MAX_DIST ) {
    Serial.println("Out of range");
  } else {
    Serial.print(cm);
    Serial.print(" cm \t");
    Serial.print(inches);
    Serial.println(" in");
  }

  // Wait at least 60ms before next measurement
  delay(60);
}
</code></pre>

<p>You can see it is used only two pins. So, as long as you <strong>can provide power</strong> to sensors and pins to it, you can have as much as you like. You can even expand with more IO pins using other electronics or go into some kind of multiplexing.</p>
