Do you have to wear a retainer if you have dentures?

No: I have not heard of wearing a "retainer" with dentures. Some people wear a nightguard to prevent tooth grinding while they sleep, but since you should take the dentures out at night, that wuldn't be a problem. A "retainer" is generally worn to prevent the movement of teeth, but you won't have that issue with a denture.

No: No you do not need a retainer, unless you mean you have partial dentures and you had braces to straighten you natural teeth before the partials were made. Then you should follow the advise of your orthodontist.

Dentures: No. A denture's teeth cannot be moved by a retainer nor will a retainer fit on top of your denture.

No: A retainer holds your natural teeth in a static position in your mouth. Your denture has teeth that are set in acrylic and will or move therefore making the need for a retainer a mute point.