I always ask myself, when writing, which word or phrase to use to describe a large number/amount of something because the English language has a huge variety of expressions as in the below examples.
a lot of, lots of, plenty of, a slew of, tons of, a mass of, masses of, loads of, a stack of, a heap of, a myriad of, myriads of, many, tens of, dozens of, hundreds of, thousands of, millions of, (and many more!)
I know that some of them just modify either countable or non-countable nouns only. Also, I have no question about the use of such phrases as "tens of," and "hundreds of" because it is self-explanatory. The phrases I'm uncertain about when to use are, for instance, a lot of, lots of, plenty of, a slew of, and tons of.
How do you choose any one among the five or probably among many more options?
JUST FOR YOUR REFERENCE
I searched Ngram Viewer for the five phrases followed by six nouns: books, money, people, water, aid, and issues. With five out of the six words, "a lot of" is most frequently used. The only exception is "water", with which "plenty of" seems to be the best match.