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 (fractions) of a bitcoin, such as bits – there are 1,000,000 bits in 1 bitcoin. Bitcoins can be divided up to 8 decimal places (0.00000001) and potentially even smaller units if that is ever required in the future as the average transaction size decreases.
For example, unlike gold or silver, should the price of either skyrocket as to make one coin extremely valuable – you could not easily shave off or measure a small speck of gold as payment for a small price purchase.  However, with Bitcoin, one could divide as small as necessary easily to determine the exact payment amount.