Parameter Field control

Hello everyone!

Does anyone know how I can disable a 'Parameter Field' from opening
when a condition is tested to be false.

My situation is that I am creating a customer shipment entry label
where by I have a detail section for quantity. You see I want to
test for how many pack number I have in the database. So if there
are more than 1 of similar pack number I would like to have
a 'Parameter Field' displayed. In the contrary, if there is only 1
packnum means that I dont need the 'Parameter Field' to get the
user's input. Therefore need to suppress the 'Parameter Field.

This is what I have written in a formula editor for a 'Formula
if Count({shipdtl.PACKNUM}) > 1 then {?Quantity}

so far I can test for the pack number although whether there is 1 or
more than 1 the 'Parameter Field' is displayed.

How do I go about controlling the 'Parameter Field' to display or
not to?

thanks in advance

NOTE: Using Crystal Reports 8.5 and viewing the label in Vantage 6.1