Stating the obvious, this depends largely on your company's size, on the use cases Cognos is applied for at your company and so on.
Yet, as a rule of thumb: up to three versions of a report are normal; up to seven might be sustainable; anything obove seven probably is paranoia driven and nobody will look at those old reports ever again. Stored versions that are older than a year probably are useless, too. In short, look a the extremes and don't mind the rest.