There are two different things, the amount of money, and the so-called “velocity” at which it is spent. If you multiply those two things together, you get the total amount of spending.
The monetary aggregates eg M2 do not make any attempt to measure velocity. They measure the money that is out there for spending. Whether it is spent, and how fast, is another question.
Many things can affect velocity such that, all other things equal, the same amount of money in the economy could lead to different rates of spending, and different rates of inflation. For instance, if people are more cautious and worried about the future, they would tend to increase the amount they wanted to save, which would reduce velocity (that’s happening right now). An example of something that might increase velocity would be concerns about the future purchasing power of the currency, which would encourage people to spend the money faster than they would have otherwise.
Both are important things to consider in your analysis. The reason that I have tended to focus more on money supply is that the various factors that influence velocity tend to be more short-lived, whereas changes to the money supply are more enduring. This is why money supply is a big factor in long-term inflation prospects, even if it is overwhelmed by velocity considerations over shorter timeframes.
