When you want to talk about a whole lot of something, but not an actual, specific number, or even an actual range, you might use a word like “zillion,” or “umpteen,” “skillion,” or “jillion.” Nowadays these “indefinite hyperbolic numerals” are used pretty much interchangeably and there isn’t much variation in which ones you tend to use based on where you’re from. But that’s a recent development — “recent,” at least, in terms of changes in language use. If you look in printed records up until about the 1940s, for example, you’ll find “jillion” in publications from the south central US, but not elsewhere. “Zillion” tended to be used in publications for African American readers. Both “zillion” and “jillion” only showed up in the early 20th century, but “umpteen” is older; it first appeared in the 1870s.
Another even more recent development in the world of indefinite hyperbolic numerals is the addition, since about the mid-1980s, of prefixes. Instead of just “zillion,” now you might use “kazillion” or “gazillion.” There are some other variations floating around as well; the main thing seems to be to end in “-illion” so whatever you come up with reminds people of actual enumeration words like “million” and “billion.”
But for that matter, why do we use “indefinite hyperbolic numerals” at all? There are already plenty of English words of long standing that indicate “many” without being specific. Like, for instance, “many,” “legion,” “countless,” “numerous,” “plentiful,” “copious,” and so on. What may be going on is that starting around the same time that “zillion” and “jillion” appeared, actual enumeration of very large numbers began to actually matter to more people. When an issue matters to more people, they adapt language to talk about it, including coining new words. The thing they needed to express was probably something like “very many of something, similar to that thing I heard about how many widgets those new-fangled factories are able to make, or how many dollars the Rockefeller family has accumulated.” Remember that although great wealth was accumulated at other times in history, now is pretty much the first era that it’s routinely measured in currency. In the past it might have been weight (of gold), size (of palaces), or appearance (of the many trappings of wealth), but not so much by a standardized number (indefinite or not) that invites comparative measurement.
In general, while languages often (but not always) have words for numbers, even for large numbers, all of that seems to come from a “number sense” that isn’t even unique to humans. A number sense doesn’t refer to counting ability, but instead to the ability to immediately recognize changes in a relatively small collection of items. A bird, for example, can recognize when one of its chicks is missing from the nest. Small children and societies that don’t use “counting on your fingers” can still easily recognize small quantities. It appears that the cutoff at which you begin to need to really count is around 4 (or so). There are some languages with minimal vocabulary around numbers, and even those typically have words for one, two, three, and “many.”
In other situations, people might have a need to express fractions in a convenient way. This is the case, for example, with Ancient Egyptian. They evidently needed to divide food and other supplies among groups of people, and they developed language and notations to communicate fractions and notations. The hieroglyph for the number 5, for example, was five vertical lines, and the reciprocal of that (one fifth), was the same five lines with a horizontal oval above it. They could calculate in fractions too; there’s an ancient document called the Rhind Mathematical Papyrus (dated to 1550 BCE) that’s a presentation of 87 math problems — all but six use fractions. By the way, there’s another one of these, the Moscow Mathematical Papyrus. It’s smaller but about 300 years older. And by “smaller,” well, this one is only about 18 feet long!
Most human languages have extensive vocabularies for enumeration and notations for writing numbers, but it turns out there are really only five types of notation systems. They just keep being reinvented over and over. The system we use today is, of course, one of the five types, and it’s well-suited to the one thing we seem to do more of now than in any previous societies we know of: writing and working with very large numbers. We need a system like that; after all, we’ve got a kajillion big numbers to deal with!