JSFiddle - React, Tailwind, and code Playground

HTML

<input id="hvorMange" type="number">
<button type="button" onclick="skrivUt12()">Finn tallet</button>
<p id="skrivUt12"></p> 


<script>
var liste = [12,14,13,76,5,1,23,12,87,124,674,98,3,7,-3,5];  


function skrivUt12(){

var tall = +document.getElementById("hvorMange").value;
var sum = 0;


for(i = 0; i < liste.length; i++) {
        if (tall === liste[i]) {
            sum++;
        }
    }
    if(sum > 0){
            document.getElementById("skrivUt12").innerHTML = "Tallet " + tall + " finnes " + sum + " antall ganger";
}else{
            document.getElementById("skrivUt12").innerHTML = "Tallet " + tall + " finnes ikke";
}
    
    
}

</script>