In preparation for our Apple (AAPL) earnings smackdown for the last fiscal quarter of 2011, which ended in September, we’ve gathered estimates from the small army of analysts — institutional and independent — who track the stock.

Last Tuesday we previewed their revenue and earnings estimates. The next day it was their iPhone unit sales numbers. Today we look at their estimates for Apple’s fastest-growing product line: The iPad.

Among the 54 analysts we’ve polled so far, the numbers range from a low of 8.78 million put forward by Wedbush’s Scott Sutherland to a high of 14.8 million iPads sold (offered by two independents, Dennis Hildebrand of Apple’s Gold and Martin Epstein on LinkedIn’s AAPL Independent Analysts).

The average estimate, 11.92 million, represents a 184.6% increase from the same quarter last year, and would bring the total number of iPads sold in 18 months to more than 40 million. The average estimate among the independents is 13.13 million. The  Wall Street consensus is 11.39 million.

We’ll find out who was closest to the mark next Tuesday at about 4:30 p.m. EDT, when the quarterly results hit the business wires. Apple is scheduled to discuss those results in a 5 p.m. EDT conference call with analysts that the company will webcast here.

Below: The estimates we’ve gathered so far and the date they were submitted, with the independents in green and the institutionals in blue. The rank numbers show how accurate each analyst’s estimates were over the past three quarters (the lower the better).