That number has been tossed around as a golden number for authors to shoot for, and many authors will go out of their way to buy, beg, borrow, cheat, and steal to get to that number of reviews. The idea is, if you get this many reviews on Amazon, you are set and your book will sell forever. There's only one problem.
It isn't true.
The thing is, reviews do matter to Amazon for determining how their algorithm rates books, and the more recent reviews are, the more they contribute toward helping your book rank higher. But, there is no golden number that Amazon looks for, or that readers look for.
The part of the process that contributes the most toward helping your book sell is having sales. The more your book is selling, the more it will continue to sell, because Amazon's algorithms are designed with one goal in mind: to sell books.
Each time Amazon loads a page on their website, it has an associated value, which in turn means it has an associated cost. Anytime a web page load contributes to a sale, they can associate that with a value. What this means is that if a book is popular and selling well, they have an incentive to get that book in front of readers and consumers so that they might purchase it.
On the other hand, if a book has fifty ratings and readers might like it, that doesn't mean readers will purchase it. So, showing such a book to a reader when it doesn't have a good track record of sales has a low incentive for Amazon because it has a high risk. To be perfectly honest, Amazon will recommend a book that is selling well but has a low number of ratings (or low ratings) over a book with a high number of positive ratings simply because the one that is selling well has already proven that it can sell.
Now, on the flip side, having reviews does influence purchasers, so it is worthwhile gathering as many honest reviews as possible. Don't try to just have positive reviews, because that makes readers suspicious. There isn't a single book in the world that is universally beloved, so if your book looks like it might be the first then people will automatically assume something fishy is going on.
The number matters way more than the rating, and books with a 3.7 rating but a lot of selling traction will move way more copies than a 5.0 book that hasn't sold a copy in months.
So, yes, you should work on getting honest reviews, but not at the expense of sales. You need to get your book selling and moving copies, and if people are reading it then the reviews will come in.
The overall net value fluctuates wildly. It used to be that one organic review came per one-hundred copies sold, and now it's closer to one review in two-hundred. Amazon is working on cleaning up their review process and making reviews easier to leave, so that number will continue to change into the future.
Don't worry too much about it. Sell your book, tell people about it, and let the reviews come naturally. Sales are way more important than reviews.