support is only my 3rd best role but i think i play it pretty damn well, so ill give u my opinion:
if you rush sightstone, the gold payoff is well worth it rather than buying wards all the time. As a support, I have always been babysitting the adc at bot lane, you rly dont need more than 2 wards at a time. 1 at river bush, and one at a bush in lane, depending on where the lane is atm. later on, when overall map vision is more important, you can upgrade and place 3 at a time, and you should not b the only one placing them, so this is fine. the gold u save can buy u plenty of aura items earlier which will greatly help your team.
2-3 wards out in the map(if upgraded).When fighting for objectives like Dragon and Baron,when a ghost ward is destroyed by Vision wards or Oracles you can just replace them(not like you can't if you have 5 wards,but there's no guarantee that you will at all times).You can buy Vision wards more often.