JUST under a hundred, means very close to a hundred, but not more than a hundred. (Obviously, it's not practical to count all of Angel's favorite books when the number can be estimated.) Tens of [something] has a similar connotation, but it's more common to say, "dozens" (12s) of [something]. It's fairly common to say, "tens of thousands" of [something], but of course that means several thousand.

“Under a hundred” here means estimating between 70-99? (Under a hundred means a number that is close to but less than 100. "Under a xx is often used when we do not have an exact number, so we use an upper limit to express how large or small a quantity is. Example: "How can a hamburger cost just under 50 cents?" In this case, the writer is questioning the possibility of a small number.) When we use “tens of” something, it means 10-99 right? (You will notice that "tens" is in the plural form. So starting from two of the number ten, which gives us 20. So the number could be somewhere from 20 to 99 because 100 would be "a hundred".) With the same analogy, "hundreds of thousands" will mean somewhere from 200,000 to 999,999.) Which is the better way to describe something possibly between 50-99? (Well, you can use "dozens of" but since the number is small and you probably know the actual number, you can just state the number. All the ways of expressing numbers are used to create a dramatic effect of a huge number of things. "Dozens of" has the same effect. They lead the reader to the idea that the quantity is mind-boggling.)

"there are possibly between 50 and 99 items" "there are between 50 and 100 (whatever in here). "there are 50-99 cars in the parking lot" you have already given the best way to describe 50-99 of anything in your question. all of these mean from 50 up to 99.