Here's my thought:
1.) They are going to just be tables: no writeups.
2.) The Greyhawk Pantheon is going to be the "3e" deity list from the PHB: 14 deities not counting the racial gods.
3.) Forgotten Realms will be the major/important Gods: probably around 20. (Lathander, Torm, Mystra, Bane, Selune, Cyric, etc)
4.) Dragonlance has a tight pantheon: 21 total.
5.) The Elves are a small pantheon: 9 deities + 6 drow deities.
6.) Dwarves: 8
7.) Halflings: 6
8.) Gnomes: 8
4-8b.) Alternately, they could do a "racial" pantheon with the heads of the major racial gods (Moradin, Correlon, Lolth, Tiamat, Eadro, Bahamut, Gruumsh, etc) probably 20 or so deities done that way. I suspect most monster deities will be in the PHB and aren't necessarily getting cleric domains.
9.) Eberron is 20 religions (22 if they add the minor faiths)
10.) Olympians: 19
11.) Pharonic: 14
12.) Asgardian: 20.
A tight font with nothing but name, alignment, portfolio, domains could easily fill 15 pages.