"Adductores magnus" (a type of thigh muscle) and "serratus magnus" (muscles at the back of the throat) are both words in the English language (in Latin they would be known as "musculus adductores magnus" and "musculus serratus magnus" respectively) and in English the plural for adductores magnus is adductores magni, and likewise the plural for serratus magnus would be serratus magni. That's in English - and obviousl it's derived from Latin, but they've maintained the same convention.

Now I've not heard of these words being pluralised as "adductores magnuses" and "serratus magnuses" in English before, so afaik the accepted plural for "magnus" in English is "magni."

It's possible that "magnuses" could be an accepted plural in English for magnus as both "octopi" and "octopuses" and "virii" and "viruses" are accepted plurals in English for "octopus" and "virus" respectively (although I've never seen "virii" used as a plural for software viruses; I've only ever seen it used in a microbiological/medical context). So I suppose it's possible that "magnuses" could be an accepted plural as well as magni - but I've yet to come across any material in English which pluralises magnus as 'magnuses.'

So afaik the only English plural for magnus that I'm currently aware of is "magni." I'll ask the other members of my faculty (and possibly the English faculty if need be) and see what they say.