Events from Dropdown w/ numeric option sets the number of text inputs

Question

Hi there,
I've a little question (a few...) about dropdown field, the events feature and syntax.
In my form, I need to set several thing, maybe a bit tricky for me (coding JS or PHP isn't my first profession...face-meh-blank)

1. Classic things I can do easily thanks to CF : create PDF with the data (hum got some problems w/ tcpdf and text formatting but it will do), save it on the server, email the data and attach the PDF. Hooray!

2. In details, the tricky part for me... : An dropdown item will set a quantity of persons ({nb}).
- This field is required to calculate a price ({nb} * constant) and the result will be recorded (I've an old javascript a friend shows me 3 or 4 years ago... But I'm sure there is something more up to date)
- I need this variable to enable / disable the same quantity of required fields (container {pers} w/ 2 text input - first name / name).
i.e. if the user sets 2 persons, I need 2 containers (ie. pers1, pers2...). 3 persons = 3 containers...
With a quantity of 2, it's easy to set the event settings, but I need at least 5 persons.

Q1 : So, is there a simple way or a syntax to set the same event (enable/disable or show/hide) for more than 1 field ? i.e.
On = 2 => ENABLE => Field PERS1 & PERS2
On = 2 => DISABLED => Field PERS3 & PERS4 & PERS5...

Q2 : If a required field is disabled, the validation script will pass through this element if empty ?
Q3 : In the same order, if a required field is set on "parent hidden", what's the effect with validation ?
Q4 : For the script I'll use to calculate the final price, it's working but if a pro could give me his discharge :

<script>
    function calculate() {
		var nb = document.getElementById('nb').value;	
		var result = document.getElementById('total');	
		var myResult = nb * 400;
		total.value = myResult;
    }
</script>
<label for="nb">Nombre de participants</label>
<select id="nb" onchange="calculate()">
<option value="">nb ?</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
</select>
Total à payer en euros :<input id="total" type="text" name="total" value="" size="6" readonly>

Many thanks for your advices!

Germain.