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?
2.7 billion market cap, not available to you. Say you needed to make a $10,000+ purchase. If you can't buy or sell that much bitcoin without considerably altering it's price it becomes a hassle of a currency.