Se doesn't distinguish number; it works both as singular and plural. In this it's different from other pronouns that form their plural with -s (there's los, las, les but there's no *ses). In this particular example se refers to the third person plural, agreeing with aceptan and tarjetas de crédito.
Since the second person pronouns usted and ustedes work grammatically as if they were third person, se can also refer to them.
se aceptan tarjetas de crédito
is not an impersonal phrase, but one of the kind called pasiva refleja ("reflexive passive"). It is equivalent to «Tarjetas de crédito son aceptadas», only that would be rather an unnatural way of expressing the idea. There are a couple of questions addressing the difference between impersonal and passive, which is sometimes hard to spot.