Is there any way (prefix, suffix, or adjective) to indicate that a noun is the most numerous in some aspect?
For instance, if I want to tell everybody that my horse carries the largest number of horns, could I call it a ____corn?
I believe the most common way to express "most numerous" is to use the superlative plurimi. For example, I might say equus tuus plurima cornua fert, "your horse carries the most horns".
Another option is to use the superlative of numerosus, "numerous". You can replace plurima with numerosissima in my example above.
The superlative in Latin is more often absolute than in English. This means that it does not express comparison ("more than any other"), but just emphasizes the adjective ("very many"). If I were given my example sentence in Latin without context, I would translate it as "your horse carries a lot of horns".
I'm not aware of a prefix with this effect, but I can coin one. However, it should be used with caution like any new coinage. For "numerous", the typical prefix is multi- and this is widely understood (in multiple languages). Correspondingly, you could use plurimi- for "most numerous". If you really need a prefix, I suggest this one, but if not, I suggest going with something more idiomatic. Somehow "multicorn" looks better than "plurimicorn" (and they both sound more like cereal brands than horn-carrying horses to me).
If the original question is about English, it's in the wrong stack. However, if you're looking for a Latin form, I'd like to offer another alternative: taking the noun, making it into an adjective with -osus, and then making that superlative.
Let's take, for example, take the word populus, "people." To describe a place as being "full of people," we have the adjective populosus, as found in Apuleius' Florida as gens populosa describing the Indians.
You can then make this superlative with regular endings, as Dicuil (and 8th century monk) did in his work on India:
In Gange insula est populosissima amplissimam continens gentem.
In the Ganges is an island most populous and containing a most abundant gens
Note: Some editions have populissima, but that looks to be a mistake.
You can do the same with other nouns. Cicero calls Apollonius, one of the plaintiffs in the case against Verres a homo pecuniosissimus (Cic. Verr. 2.5.9), literally a "man most full of money" (another way of saying very wealthy). This construction goes back to the very earliest Latin prose we have, beginning with Cato's aquosissimus.
Unfortunately, for your specific example, there are no attested forms in Classical Latin, but were someone to write cornuosus and cornuosissimus, an ancient Roman would have understood what it meant.
If words that combine elements from Latin and Greek are allowed, there's the fairly prolific Greek prefix myri(a)-, which literally means '10,000' but is used in a general way for very large numbers, in the sense 'countless'. This would give myriacorn.