Fortunately your Add to Cart code is already using Variants, so it's relatively simple to add a 'quantity' field -- <input type="text" name="quantity"/> -- inside your button's FORM section:
<form action="https://www.e-junkie.com/ecom/gb.php?c=cart&i=840887&cl=143487&ejc=2" target="ejejc" method="POST" accept-charset="UTF-8">
Date/Time:<br/><br/>
<select name="o1">
<option value="January 14 at 7pm">January 14 at 7pm</option>
<option value="January 15 at 3pm">January 15 at 3pm</option>
<option value="January 15 at 7pm">January 15 at 7pm</option>
<option value="January 20 at 7pm">January 20 at 7pm</option>
<option value="January 21 at 7pm">January 21 at 7pm</option>
<option value="January 22 at 3pm">January 22 at 3pm</option>
<option value="January 22 at 7pm">January 22 at 7pm</option>
<option value="January 23 at 3pm">January 23 at 3pm</option>
</select><br/>
Ticket Type:<br/>
<select name="o2">
<option value="General Admission: $18 + $1 fee">General Admission: $18 + $1 fee</option>
<option value="Senior/Student/Child: $15 + $1 fee">Senior/Student/Child: $15 + $1 fee</option>
</select><br/>
Number of seats:<br/>
<input type="text" name="quantity"/><br/>
<input type="image" src="http://www.e-junkie.com/ej/ejadd_to_cart.gif" border="0" alt="Add to Cart" class="ec_ejc_thkbx" onClick="javascript:return EJEJC_lc(this.parentNode);"/>
</form>