What is the plural of "corpus callosum"?

Corpora callosa is the plural form.

See where it says "plural" here or here.

You almost had it right: it should be several corpora callosa in Latin. If you are transplanting the whole thing into English without naturalizing it as in this case indeed applies, then just leave it like that.

The Latin -a plural is for neuters, not just ones like callosum > callosa and datum > data in the second declension but also neuters in other declensions, like nomen > nomina and carmen > carmina in the third or cornu > cornua in the fourth.

But corpus although neuter is not in the second declension, so it does not become “corpa”.

Rather, it is in the third-declension where the neuter corpus/-oris is to be found, and so its plural is corpora — even in (scholarly) English use.

Some people do say corpuses in English, which is perfectly normal and fine, but if you are going to use the full Latin phrase as you are doing here, you can’t cut corners by using English inflections in one place and Latin ones in another.

Other third-declension neuters you might recognize include:

• opus > opera
• genus > genera
• tempus > tempora

We don’t use tempora much in English, but it did gave us our adjective temporal. So you can find vestiges of the third declension showing up in derived forms in English, like in English general and corporal.

Other examples of third-declension nouns giving us adjectives include pectus/-oris which gave us English pectoral and also Venus/-eris which gave us English venereal.

_{(Not that Venus is neuter; she is of course feminine. But she is third declension.)}