Ordinals are numbers assigned to a list of inputs (e.g. first, second, third, etc.). In bitcoin-context, ordinals are identifiers/serial numbers of each satoshi, or sat, which is the atomic unit of each bitcoin. Each bitcoin is made up of 100M satoshis. So with 19.1M bitcoins in circulation, there are 2 quadrillion sats.

Each satoshi’s ordinal would be a number between 0 and 2×1010, depending on when it was minted. For example, the 500th satoshi would have an ordinal number of 500