Within the first two months after your book's release, you should aim for at least 20 reviews. This demonstrates that your book has a following among genuine people.
You're probably fine to go after roughly 50 reviews. You've established the book's credibility at this stage, and it should continue to receive positive feedback.
Amazon is an intelligent company. Their algorithm isn't a joke, and you'll be caught if you try to cheat them. They go out of their way to discover and remove any review that a real person doesn't make, and they're excellent at it.
Amazon also has a strict no-tolerance policy regarding reviews that are intended to deceive or influence buyers. They don't hold back when it comes to eliminating reviews that breach the guideline.
Here's a list of things by to stay away from:
There is no way to pay for reviews. This can be in the form of money or a free or reduced good.
You can't give reviewers presents in return for the positive feedback.
You can't swap favorable book reviews with other authors, so don't look for review exchanges on Facebook or Twitter.
This may appear to be somewhat restricted. After all, don't we rely on our business associates, friends, and other network members to sell our books? What about providing complimentary review copies? Is this a violation of the promotion's terms?