At the moment of this writing, there are 13725450 Bitcoins in existence [1], with a value of $195.30 each [2]. That makes a grand total of $2,680,580,385 (2.6 billion).

How low would they have to fall for there not being enough of them to perform a transaction?

[1] https://blockexplorer.com/q/totalbc [2] https://www.coinbase.com/charts