<label for="first-name">Your phone number:</label><br />

<input id="first-name" type="tel" name="phone" minlength="9" maxlength="9" placeholder="It must be 9 digits" />

input {
  margin-top: 0.5rem;
}

input::placeholder {
  font-weight: bold;
  opacity: 0.5;
  color: red;
}