If you hit ESC and go into Options - Video Settings, you can change the "GUI Scale".
On "Small", in Monster, NEI is 7 pages and there are so many icons on my screen (I run at 2560x1600 lol) I cannot possibly count them. Thousands.
On "Normal", NEI is 29 pages. I fit about 40 rows of 30 items on each page.
On "Large", NEI is 72 pages. 27 rows of 18 item icons.
On "Auto", NEI is 444 pages, only 13 rows of 6 items on each page.
Using those numbers, there are approx. 13*6*444 = 34,632 items in NEI, in Monster.
So on Small GUI, I can fit about 5000 items per NEI page on one screen. LOL By the way... it really bogs my rig down a lot when I do it. That's why I usually run on Auto believe it or not, despite the big screen. Small icons are just tough to distinguish. Large setting is playable, anything below that is not (for me anyway). Really slows the computer down.