You will have to include some javascript to set the input max / min attribute.
<script>
window.onload = function() {
var select = document.getElementById('budget');
select .onchange = function() {
switch(select .selectedIndex) {
case 0:
SetMinMax(500,1000)
break;
}
}
}
function SetMinMax(min,max){
document.getElementByID("amount").setAttribute("min", min);
document.getElementByID("amount").setAttribute("max", max);
}
</script>
<label for="budget">Category Budget</label>
<select id="budget" name="budget">
<option value="select">Please Select<option>
<option value="take_it_easy">Take It Easy: RM 500 - RM 1000</option>
<option value="not_too_much">Not to Little Not to Much: RM 1000 - RM 5000</option>
<option value="Do_as_much_as_possible">Do As Much As Possible: RM 5000 - and above</option>
</select>
<label for="amount">Your Amount</label>
<input type="number" id="amount" name="amount" required="required" placeholder="Eg: RM 1000">
That should get you started. You may also need to check if the input already has a value outside of the range as it may not automatically change.
Hope that helps ^_^