Bitcoin’s supply is capped at 21 million units, a characteristic that has drawn the interest of countless investors as evidenced by the more than $25 billion flowing into spot exchange-traded funds (ETFs) since the beginning of the year. This surge in demand is occuring at a time when bitcoin’s supply is becoming increasingly constrained. More than 19 million units of the digital currency have been mined and the percentage of bitcoin available for trading on exchanges recently fell to the lowest amount in more than six years, according to Glassnode data.