Brennin Magalus said:
Up to four for a particular cleric/divine caster seems fine to me, but more than four seems excessive. What do you think?
I would say that the max number of domain for an individual Cleric should be in line with the domains of her deity. If you have very many deities in a campaign, they probably get a few domains each (like the GH or FR deities), which may reflects the fact that a large number of deities has to divide among themselves the existing portfolios. If you had a campaign with only 4 gods, they would probably hold many domains each, and possibly with less overlaps, in which case a Cleric of a single deity may be granted access to more domains herself.
IMHO, a Cleric of a god is always more or less concerned with the whole portfolio of her deity. For example, a Cleric of Obad-Hai would probably enjoy all 4 elements AND animals AND plants. For balance reasons, the rules give her only 2 domain powers & spells, but this doesn't mean that she doesn't care for the other interests/aspects of her deity.
For these reasons, from a RP point of view I don't see restrictions on how many of these could be a Cleric's interests. If you are questioning instead how may should a class get before being too much power-wise, well of course it depends on what else the class/PrCl gets. Remember that every domain gives an extra power, while domain spells per day are always the same number (although the more the domains, the broader the choice). Domain powers are IMHO usually definitely better than a feat, the weakest of them giving a bonus feat exactly, so I would try to limit them to about 2 (to begin with) plus 1 every 5 levels, in the best case!