In the current view of galaxy formation and cosmology most galaxies undergo phases of high nuclear activity, where they accrete lots of matter (compared to their Eddington accretion rate) and shine brightly. Such an object is then called an Active Galaxy and the thing in the center that actually shines and where lots of other stuff is happening is called an Active Galactic Nucleus or AGN.
Now how would one call two such objects? Certainly they are Active Galactic Nuclei, since the latin word nucleus (Second declension, “o-Deklination”) has a plural ending of -i in the nominative.
However, this does not mean that the plural of AGN necessarily must be AGN, AGNi or some other funny, but counter-intuitive, abbreviation.
also a the only grammatically valid plural form of AGN. This is because the term AGN is so widely used that it itself has actually become lexicalized, i.e. it has become a word by itself. In other words: People actually say “The galaxy has an A-G-N” (instead of “The galaxy has an Active Galactic Nucleus”). And since standard English words get an ‘s’ in the plural, the plural of AGN is therefore AGNs.