Generally speaking, yes, fifty copies a day will get a book into the low three digits, but there are many variables at work, and usually the lowest ranking number your book gets on amazon is the accurate number.
And for the low ranking to mean anything, the book must sell copies day after day after day. You can, for example, get a quick and dramatic jump in sales rank by having friends all buy books on the ame day, but the number will be meaningless unless the sales continue hour after hour and day after day.
I don't have numbers for what the bestsellers sell each day on Amazon, but the top sellers are updated hourly, which is one big reason why the off book that sells fifty copies on only one day means nohing. It will nearly always be knocked out of the ranking before the day is over by books that are true bestsellers.
In the end, of course, it isn't sales rank on Amazon that matters to anyone, it's how many copies your novel actually sells overall. Some books with pretty high rankings on Amazon sell very few copies elsewhere, which means the publisher still loses a lot of money on the book.
A book from a mainstream publisher often has to sell a thousand copies a week, or have a total of 30,000-50,000 sales, before the publisher will even put any publicity money behind the book.
