Neznajka,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script>
document.addEventListener("DOMContentLoaded", function() {
let ar = Array.from(document.querySelectorAll("#cTabZkp [name='Prd']"));
let length = ar.filter(({ value }) => !value).length;
alert(length);
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</script>
</head>
<body>
<table id="cTabZkp">
<thead>
<tr>
<th>X</th>
<th>N#</th>
<th>Product</th>
<th>Number</th>
</tr>
</thead>
<tbody>
<tr>
<td><button>X</button></td>
<td>1</td>
<td><input type="text" name="Prd" value="medium MCDB"></td>
<td><input type="text" name="Nkt" value="20483020"></td>
</tr>
<tr>
<td><button>X</button></td>
<td>2</td>
<td><input type="text" name="Prd" value="cell tracker"></td>
<td><input type="text" name="Nkt" value="D8026"></td>
</tr>
<tr>
<td><button>X</button></td>
<td>3</td>
<td><input type="text" name="Prd" value=""></td>
<td><input type="text" name="Nkt" value=""></td>
</tr>
<tr>
<td><button>X</button></td>
<td>4</td>
<td><input type="text" name="Prd" value=""></td>
<td><input type="text" name="Nkt" value=""></td>
</tr>
<tr>
<td><button>X</button></td>
<td>5</td>
<td><input type="text" name="Prd" value=""></td>
<td><input type="text" name="Nkt" value=""></td>
</tr>
</tbody>
</table>
</body>
</html>