I don't think there is any doubt that a "case" means twelve bottles.

However, in the retail market, it is quite common to refer to, for example, "a case of three bottles". That most traditional of wine merchants, BBR, even has the absurdity of a "mixed case of one bottle" on its website.

Was this case part of a compensation deal to which you were asked to agree (presumably you were then also told what the wine would be)? In that case I think you were duped. But wine is usually sent as a conciliatory gesture rather than part of an agreement - I got six bottles of decent-ish wine out of a well-known Edinburgh-based bank once - so you can't really complain, even if it leaves you feeling a little disgruntled.

Anyway, what wine did they send? If it were, say, three bottles of Yquem 1921 then I'd consider adequate compensation for a ruined RTW trip in F.