How many gp10 items for support?

[Updated for Season 3]

The game is currently in flux (season 3 has only been out for a couple days), but with the boosted ambient gold gain, it appears you only need one gp/10. I've been building Philo Stone to great success. If you expect to get some significant assist-gold (you're confident in your kill lane, or you're Soraka and have a good eye for your ultimate), you could probably stand to buy a Kage's Lucky Pick, but I much prefer to only buy the one Philo Stone and start immediately after into my build (Sightstone, Shurelya's, Aegis -> Bulwark).

Again though, this is all pretty fresh. Your mileage may vary.

[NOTE: The answer below was provided for season 2. There were significant changes implemented for Season 3, and this answer is now out of date. It is provided here for history, but should not be followed.]

It depends. The rule of thumb is two (Philo Stone and Heart of Gold), and 98.355% of the games, you'll want those. If the game is going to absolute crap, teamfights started early and your team is getting routed, you may have to skip the second and start pouring that extra money into wards and an oracle's (counter to what you'd think, oracle's is a great purchase if you're falling behind and the laning phase is over).

I'd be very suspicious of my support if they got a third gp/5. The only real time I can see that being worthwhile is if you know it'll be a late, late game, and that you'll need to be a significant contributor in teamfights for damage.

The first thing about gp10 item is to know that you get very little stats for the money.

It is a good idea to get two asap if you are doing well in your lane. That is because with little effort you are still winning, and in the long run your gp10 will have a better benefit than your stat items.

Typically you want to try and get your first philostone quickly even before your boots because it will increase your money in the long run.

If your lane is doing well get that second gp10 before boots! If your lane is getting ganked often or they put too much pressure you may need to buy boots instead.

The only reason to get the third or fourth gp10 is if your team is doing very well, where you almost have no effect at all. An example is where your AD was fed 3+ kills and you can still focus on gp10 because they will carry you with minimal effort. In the long run this will grant you more money. However if you decide to take better items instead (aegis), you will be more helpful in team fights.

I have played as soraka where I had 4 gp10 items, boots, a stack of wards, oracle, and plenty of money saved up. The enemy team has almost no vision while their whole jungle was warded.

I have also played as soraka where I needed to get more support items to stay competitive.

Overall, this depends on the situation.

You should try to experiment and get a feel for what works best with your play style.