League of Legends (LoL) Question: How many supports should I start ranked ?

From a support main; focus on BIG plays champions. Such as Bard/Thresh/Braum/ something that even if your adc is the dumbest human being alive you can still peel/save them and live through it. Being a Mastery 7 Bard main carried me my first season to plat 4 alone

TL;DR is: Epic Fail Guy is right; focus on three or four champs that fit your playstyle (Poke/Heal) and grind.

Long version:
First, figure out your support playstyle: are you poke-y (primarily AP champs sent bot-lane), heal-y, or tank-y? These are the three primary support categories, and they work in a rock-paper-scissors formation: Poke supports don't do well against Tanks, Tanks can't out-sustain Heals, and Heals are weak against Pokes.

Next, coordinate with your ADC on who they're going to play. Generally all ADCs have no ability to escape engages on their own, so the Tank support is pretty dominant this season, but the Poke and Heals work with different ADC playstyles.
For example, if my ADC is playing Tristana, I know Tristana pretty heavily relies on her opponents to stay at mid-range in order to do the most damage, so I go Leona or Thresh for their CC. However, I can also go Morgana or someone with a snare that is comfortable with going deep into an engagement to deal damage equally with Trist (Poke), or Janna, who can give Tristana buffs and stun opponents with her tornado (Heal).

Lastly, follow Epic Fail Guy's advice and stick with playing only 4 or 5 supports, instead of buying as many as you can to be "flexible." Pay attention to who gets banned a lot and who gets played a lot, and choose to play champs that aren't in either of those categories. For example, I've noticed that Morgana and Zyra tend to get banned very frequently, and Leona, Soraka, and Karma get picked frequently, but few people play Braum, Janna, or Lulu. Because of my playstyle (and his epicness), I frequently play Braum, and subsequently have become very good at him. Janna and Lulu are difficult, mechanics-wise, but are good examples of Heal and Poke supports to play.

As far as recommendations, I'd buy Braum (tank), Morgana (Poke) and Sona (Heals), as they're relatively straightforward. I'd also recommend Soraka, but she gets banned every other game, so it's not a safe investment.

Hope this blurb helps!

There are no required amount of support to play ranked, but I would recommend focusing on 3 supports to play. That way you can focus on improving them and making those LCS plays.

As for which supports? - At the moment Thresh, Janna, Nami, Alistar are the best in my opinion. But, if you're low ELO I wouldn't really play champs if they're strong in the current meta, as you get to gold + then you should start playing strong picks

Hope this helped :)