How many control wards should you buy as a support on average?

As a support I always try to provide as much vision as possible, and place an average of 6-7 control wards per game. Meanwhile when I check other people's profile from the same elo, or my friends' match history, they always only buy 0-3 control wards every game. Which made me wonder. Am I the one investing too much into vision, or are they the ones who don't invest enough? For me vision seems very strong and I enjoy being able to have control wards on my hands, but it just feels so weird to me that every support I see in game buys less than half as many wards as I do, which makes me think maybe I'm doing something wrong. Is there an optimal amount of control wards let's say in a 25 minute long game?
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