How many words ending in latin um suffix?

In Latin, the -um suffix has three basic functions:

1. it marks the nominative/vocative singular (subject) form of second-declension neuter nouns and adjectives
2. it marks the accusative singular (direct object) form of all second-declension nouns and adjectives regardless of gender, of some masculine pronouns, and of fourth-declension nouns
3. it marks the genitive plural (possessive) form of all nouns, adjectives and pronouns (by itself or as part of the suffixes -arum, -orum, -ium, -uum and -erum)

This means that every Latin noun, pronoun and adjective (except a few that have no plural forms at all) has at least one form that ends in -um. However, if the question is about Latin Words borrowed into English, then category 1 above is the relevant one, because nouns tend to be borrowed in their nominative form (unless they're part of a borrowed phrase). It's hard to say exactly how many 2nd-declension neuters there are in Latin, but as a rough guide, the dictionary file for William Whitaker's Wordsprogram has 3401 such Words out of a total vocabulary of 37,978, which works out to just under 9 percent.