Having looked over my copy of The Silmarillion I did some minor tinkering. I decided the 3 most powerful Gods (Manwe, Ulmo, Aule) should have 5 domains and the rest of the Aratar (The High Ones of Arda) should have 4 while the remainder should have 3. Exceptions are Nienna (who is considered powerful but I find it hard to find appropriate domains) and Tulkas (who I've given 4 domains). Melkor gets lots but they all seem proper. I also tried to make sure all the domains were used without too much overlap.
God............Alignment...............Domains
Manwe.....Lawful Good...........Air, Good, Knowledge, Law, Sun
Varda.......Neutral Good..........Good, Knowledge, Luck, Sun
Ulmo.......Chaotic Good...........Chaos, Good, Strength, Travel, Water
Aule........Lawful Good............Earth, Fire, Law, Magic, Protection
Yavanna..Neutral Good...........Animal, Earth, Good, Plant
Namo........Neutral.................Death, Knowledge, Protection, Trickery
Nienna.....Neutral Good...........Earth, Good, Knowledge
Orome.....Chaotic Good...........Animal, Plant, Strength, War (Spear)
Vana.......Neutral Good...........Good, Luck, Plant
Tulkas.....Lawful Good............Good, Law, Strength, War (Longsword)
Nessa.....Chaotic Good...........Animal, Good, Travel
Irmo........Neutral Good...........Magic, Protection, Sun
Este........Neutral Good...........Good, Healing, Luck
Vaire........Neutral..................Knowledge, Luck, Trickery
Melkor.......Neutral Evil.....Chaos, Death, Destruction, Earth, Evil, Fire, Magic, Trickery, War
