Would this mean that I've never been to a bad party so I can't recall one, or bad parties I've went to don't stay in my memory.
Rather the first one, although with a vague 'a bad party' the second option is probably okay as well.
I checked the given translation of imprezy and it gave parties. Why is it now singular?
Depends on the case. As most feminine nouns, "impreza" has the following cases identical: Nominative plural, Accusative plural, Genitive singular.
"pamiętać" (to remember) takes Accusative. It's negated here, so it takes Genitive instead (the only case which changes when negated). As we know it's Genitive, then it can only be singular.
The jokes posted on comments are a bit annoying ... some times I am in a hurry to complete the daily lessons and I spend the time reading comments in order to learn, not be entertained. So sorry to be harsh but it is a fact.