The plural is also "carp.' But if you have more than one carp, then you may have a carp problem. (That was a joke about carp.)

The noun or the verb? :-) https://learnersdictionary.com/definition/carp

You should have asked "Is it still carp or (is it) carps?" You are Polish. Are you Polish or Lithuanian? They are crazy. Are they crazy? That is correct. Is that correct?

The plural of carp is still carp. It is a type of noun called "uncountable" in grammar books. Other examples are fish, water, snow, etc. where the singular and plural are the same. Of course, you can have "fishes" and "waters" but only in specific instances for example "the waters parted and allowed the Israelites to cross the river".

Carp