<p id="hitung">
    0
</p>

<button class="btn btn-primary" id="add" onclick="add()">Hitung</button>

<script>
    function add()
    {

        var hitung = document.getElementById('hitung');

        document.getElementById('hitung').innerHTML = '';
        var jumlah = hitung.textContent += 1;
    }
</script>