Where is the info on how many copies it (or any D&D rule book) sold? I've never seen numbers, but reading between the lines of various information here and there, it was my understanding that the Moldvay set was far and away a better seller than the Holmes set.
For the record, (because I know my asking the above will be taken harshly if I don't clarify), I'm not challenging or denying your assertion, I'm just asking for the information that supports it.
Not at all, it's a fair question.
First of all, there's the point that the Holmes Basic set instigated the entire D&D line, separate from AD&D. Instead of just an intro to AD&D, TSR decided to create the Expert and Companion sets as a companion to the revised Basic. Note that the Cook/Marsh Expert set was released at the same time as Moldvay Basic, so it wasn't a reaction to the sales of Moldvay, but rather to Holmes. This
quote from Gygax also supports that the success of the Holmes Basic inspired them to create an Expert Set (which then led to the Moldvay/Cook/March revision). That post also suggests that, contrary to popular thought, the D&D line was not just kept around due to the Arneson lawsuit, but because it was a big seller.
JeffB has mentioned Holmes Basic had 7 printings. So what does that mean for sales? Well, in this
article in 1979, Gygax says the Holmes Basic is selling 4,000 copies a month. This is compared to probably 5,000-6,000 copies of the original D&D set for all of 1974-1975 (including Gygax's estimates for pirated copies). Then in this
interview a year later, Gygax says it's selling 12,000 a month. These were tremendous numbers for the fledgling RPG industry. Heck, pretty good numbers even today.
Here are some more contemporary Gygax writings on the Holmes Basic, and
here's a collection of Gygax recollections in recent years, just for some historical background.
Edit: It is indeed my understanding that Moldvay sold even better than Holmes (and later Mentzer even better than Moldvay). But not selling better than Moldvay doesn't mean that Holmes did not do very well.