The Acaeum Monster Manual page has this listed under the first print of the Monster Manual (Dec 1977):

"Gygax has stated that this first print numbered 50,000 copies"

The source for this info is not listed.

http://www.acaeum.com/ddindexes/setpages/monster.html

The Acaeum Print Run estimate page has the same info, and various estimates for other products:
http://www.acaeum.com/library/printrun.html

In Dragon, Gygax revealed that the first print of the MM was not received until after Christmas, so it may not have actually been available until Jan 1978.

Zenopus wrote:In Dragon, Gygax revealed that the first print of the MM was not received until after Christmas, so it may not have actually been available until Jan 1978.

It didn't become available here (Alabama) until the early months of 1978.  
So, the statement above is consistent with what I remember.

Keith