It depends on when you count. Here are the number of characters included in dictionaries and word lists from various periods:

1000 BC 4,500 jiaguwen
100 AD 9,353 Shuowen
1066 31,319 Leipian
1716 47,035 Kangxi zidian
1990 54,678 Hanyu da zidian
1994 85,568 Zhonghua zihai

Other interesting statistics:

* Number of different characters in the Confucian Analects (Lunyu): 1,382
* Number of characters mastered by a traditional scholar: 6,000-7,000
* Number of characters required for adult literacy in China today: 3,000
* Number of characters required of primary school children by age of 12: 2,535
* Number of characters required to comprehend 99% of modern printed materials: 3,500
* Number of characters required to comprehend 99.99%: 5,200

And a shameless plug:

* Best way to estimate how many characters you know (up to 3500): the Chinese Character Test on the Clavis Sinica website