Templates give a company the ability to create larger numbers of advertisements much faster than they would otherwise be able to. Rather than have to create a completely new ad each time, they can bring out existing templates to keep the process simple and easier.
One potential problem, however, is the risk that people will begin to notice and dislike the repetition that will naturally develop if one is used too often. For some forms of advertising, this does not matter quite as much, because they are long enough to allow you to make several visual changes to mask the fact that the template is the same.
With postcard templates, you do not have this same luxury.
Because a postcard is so small, you have fewer things you can change about it. Your customers will quickly notice a single template because of the repetition it will create. Most only have three or four different things on the front of it. Just rearranging them will not lead to a wide variety.
This leads to one of the pros and cons of postcard templates. The con is what I just listed, but the pro is that postcards are naturally easy to design.
This means you can more conveniently create three or four different templates, if not more, to cycle between. Going with a system like this you can have distinctively different ones for each of your postcards, and then within each, you can make things more varied by altering where you put images and text each time you use them.
The best kinds of templates are the ones that will give you a large amount of flexibility in terms of what goes where on the postcard. The more variation you can get out of a single template the better it will serve you, which is why I would suggest spending a lot more time designing it than you would normally spend on just a single postcard.
The number of templates you need, along with how often you update them, will depend entirely on how often you send them out.
Remember too that the more complicated and unique your template is the easier it will be to spot again when you only change around a few things. Good ones lend themselves well to multiple arrangements without tipping people off to the fact that it is the same. Be creative, but do not get so carried away you end up with one that is only meant for a single situation.