The more people who use proof of stake blockchains the more power it will require, same as the more people who visit CNN.com or the more people who turn on a light bulb, but it's not nearly as much as proof of work.

The difference between proof of work and proof of stake is that the first one in order to function requires showing that someone did a bunch of processing on their computer. By attaching a financial cost (literally expending energy) to mining new coins, PoW helps avoid someone fraudulently taking over the network and issuing as many new coins as they want.

Proof of stake works by people who already hold coins putting some of them up as a kind of collateral, so it's nowhere near as processing intensive.