document.getElementById('generateBtn').addEventListener("click", numGenerator)

count = document.getElementById('counter').value,
 min = document.getElementById('min_number').value,
 max = document.getElementById('max_number').value;

 (note: those should also be semicolons and not commas at the end)

document.getElementById('generateBtn').onclick = function() {
  var newCount = parseInt(document.getElementById('counter').value);
  var newMin = parseInt(document.getElementById('min_number').value);
  var newMax = parseInt(document.getElementById('max_number').value);
  numGenerator(newCount, newMin, newMax);
};