The number of pages varies depending on which encyclopedia you mean and what year it is. World Book Encyclopedia 2016 has 22 volumes. Encyclopaedia Britannica 2016 has 30 volumes.
The Otaku Encyclopedia has 248 pages.
Encyclopedia Brunoniana has 629 pages.
Encyclopedia of Conifers has 1500 pages.
Encyclopedia of Chicago has 1117 pages.
Dinosaur Encyclopedia has 376 pages.
Armenian Encyclopedia has 720 pages.
Conspiracy Encyclopedia has 320 pages.
The Encyclopedia of Malaysia has 144 pages.
WWE Encyclopedia has 366 pages.
Encyclopedia Horrifica has 144 pages.
It isn't possible to say how many pages are in any one encyclopedia without knowing which encyclopedia, and which edition of that encyclopedia, is of interest.
The Sotheby's Wine Encyclopedia has 664 pages.