Did you ever focus on the term -Liquidity- that is generally used to understand the flow of forex market?
For the sake of simplicity let us say Liquidity defines the volume of transactions that happens per day, in the sense of how many transactions and how much of amount is being traded. Liquidity defines the ease with which you would be able to carry out orders in the market, without even manipulating the prices. s.
If we take a rough idea then number of traders must be twice of the number of transactions in ideal case, if one buyer and one seller are involved in one transaction. But actually single trader trades over multiple currencies which results more than one transaction. In short, liquidity defines the volatility of the price movements. The more any currency pair is liquid, the less it will move since large volume of trades are executing on that pair. The less liquidity means harder to sell or buy that pair.