I'm not sure I understand your question - what do you mean by minimum domain - the minimum number of domains? If so, one is enough if it is saleable! If you are planning to set up a website etc. it probably makes sense to have at least a dozen or more though.
Probably it is better to get your domain out there, rather than email directly people not looking for a domain name. Of course if it is local in nature and you know the probable buyers that is different.