I succeeded, but I had to hand-code everything. Unfortunately, I do not understand how I could use WA for this. Below is the code for the select list that now works as I wish:
<?php $startnr = 1; ?>
<!-- *** Antal *** -->
<div class="form-group">
<label for="Antal"><i class="fas fa-star-of-life"></i> Antal personer (max <?php echo($WADABokning_update->getColumnVal("PlatserKvar")); ?> st)</label>
<select name="Antal" required="required" class="form-control" id="Antal" onChange="KW_calcForm('Summa',1,0,'#Antal','*','#Prisklass')">
<option value="">Välj antal</option>
<?php while($startnr <= $WADABokning_update->getColumnVal("PlatserKvar")) { ?>
<option value="<?php echo "$startnr"; ?>"><?php echo "$startnr"; ?></option>
<?php
$startnr++;
}
?>
</select>
<div class="invalid-feedback">Vänligen fyll i antal platser</div>
</div>