How many iPhones did Apple sell last quarter?

No product contributes more to Apple’s (AAPL) bottom line than the iPhone.

In the 2nd fiscal quarter that ended in March, when the company sold a record 18.65 million units, the iPhone and related products and services generated $12.3 billion in revenue, almost exactly 50% of Apple’s total sales for the quarter — more than the Mac, iPod and iPad combined.

So when Apple reports its 3rd quarter earnings next week, everybody who follows the company — from its most steadfast investors to its fiercest competitors — will be paying particular attention to how many iPhones it sold.

The estimates of the 39 analysts we polled for our quarterly earnings smackdown were all over the lot, from a high of 20.25 million from Traderhood‘s Nicolae Mihalache, a Romanian mathematician who teaches at the University of Paris and tends to overestimate, to 15 million from Needham’s Charlie Wolf, a veteran Apple hand who tends to come in low.

The average all the estimates, 16.9 million, represents a 101% increase over the same quarter last year, and would bring the total number of iPhones sold over the past four years to more than 125 million.

The 39 analysts in our survey included 14 amateurs and 25 professionals. Their average estimates were closer than in past quarters. Among the amateurs it was 17.8 million. Among the pros, 16.4 million. We’ll find out whose estimates were closest to the mark when Apple reports its earnings next Tuesday.

Below: The estimates we’ve gathered so far and the dates they were submitted. The Q2 rank numbers show how accurate each analyst’s estimates were last quarter (the lower the better).

[UPDATE: Three more analysts have come in with iPhone numbers since this was posted: Wedbush’s Scott Sutherland (18.65 million), UBS’s Maynard Um (18.35) and CitiGroup’s Richard Gardner (18 million). That brings the Wall Street consensus up to 16.6 million and puts the overall average above the 17 million mark.]