#number {
      animation: stretchdelay 3s infinite ease-in-out;
      font-size: 35px;
      font-weight: bold;
      color: orange;
    }

    @keyframes stretchdelay {
    0% {
      opacity: 1;
    }
    100% {
      opacity: 0;
    }

    }

let number = 381;
let period = 3000;
let pn = randomInteger(1, 5);
let n = document.getElementById("number");

function numberplus() {
  number += pn;
  n.innerHTML = number;
}

function randomInteger(min, max) {
  var rand = min - 0.5 + Math.random() * (max - min + 1);
  rand = Math.round(rand);
  return rand;
}
setInterval(numberplus, period);
n.innerHTML = number;