There's no general rule about plural nouns in Spanish being translated as singular nouns in English or vice versa.
With the particular example copa de bodas, what's happening is that, for some reason, the names of some ceremonies are often pluralized, so you have bodas "wedding", funerales "funeral", etc., though that's a bit old-fashioned. (The well-known opera "The Marriage of Figaro" is known as Las bodas de Fígaro in Spanish. Gabriel García Márquez's story colection Los funerales de la Mamá Grande is translated as "Big Mama's Funeral"; it's just one funeral!)
Loose compounds of the type of "wedding cup" cannot be used to infer a rule, because in the vast majority of cases the first noun is always singular even though the meaning implies plurality. A "bear hunter" doesn't stop at one bear; a "fish tank" can and usually does contain more than one fish; an "office building" is by definition made up of many offices. This has to do with the way in which English treats nouns when they act as modifiers, not with the nouns themselves or their grammatical number.
There are comparable rules in Spanish for when a noun forms a compound. For example, boca "mouth" becomes boqui- in boquiabierto "open-mouthed", and punta "[pencil] point" becomes -puntas in sacapuntas "pencil sharpener". The latter of these can be an example of nouns becoming plural in Spanish when in English their translation would be singular (or rather, would look like a singular noun). Other examples are lavarropas "washing machine" (from ropa "clothing") and mondadientes "toothpick". But these, again, are not examples of a hypothetical rule that would let you know how to translate a noun between Spanish and English (as you can see from the examples themselves).