Bitcoin is unique in that only 21 million bitcoins will ever be created. However, this will never be a limitation because transactions can be denominated in smaller sub-units of a bitcoin, such as bits - there are 1,000,000 bits in 1 bitcoin. Bitcoins can be divided up to 8 decimal places (0.000 000 01) and potentially even smaller units if that is ever required in the future as the average transaction size decreases.
