"You like very many fish."
Translation:Plurimi pisces tibi placent.
plurimi here is nominative plural case (nominative case being the subject of the sentence, the one 'doing' the 'verb').
plurimos would be the accusative plural case (the accusative is often the direct object, the one the 'verb' is 'performed' upon).
Since, plurimi is an adjective it has to agree in case with the noun it modifies. Here, pisces.