Training corpuses (corpii?) have always had lies, propaganda, derangement, you name it.
The Latin plural of corpus is corpora. It gets used a lot in linguistics articles, perhaps other fields as well.
However given that in English we don't care eg about Latin cases, I don't think we should care all that much about Latin plurals. So corpuses is fine by me.
"Corpi" gets used as a hypercorrection, but it's deprecated in favor of the two previous.
"Corpii", as suggested above, is right out.
I often use corpora, because it's what's common enough. And I sometimes use nonsense plurals like octopodes just to be silly.
The reason I replied is because the OP appeared to be interested in the most accepted or used plurals for this word.