If everyone on earth took a high dose of lithium carbonate of 1000mg a day, that would be 7000 tonnes a day. It's 14/74 lithium by weight, so around 1300 tonnes of lithium metal per day. Total lithium mine extraction is 180,000 tonnes a year, so 500 tonnes a day.

So assuming everyone on earth was fully dosed up, and you could collect all the waste, it's actually a lot!

The actual figure of 2 million people in the US on lithium carbonate, probably on smaller doses, is much less interesting: under 2000kg of pills, "only" a few hundred kilos of pure lithium per day. Though I think still interesting scale illustration that one pill each for 0.5% of people in the country weighs about the same as a car.