Four.
The first joints in all fingers on his left hand, except the thumb.
“Is it true my uncle Stannis cut off your fingers?”
“Only the last joint. I still have fingers, only shorter.”
“Show me.”
Davos peeled his glove off. The boy studied his hand carefully. “He did not shorten your thumb?”
“No.” Davos coughed. “No, he left me that.”
—- Edric Storm and Davos Seaworth, A Storm of Swords
In the TV series, Liam Cunningham, the actor who plays Davos, is left-handed, so it was his right hand that was maimed. Additionally, it looked like two joints from each finger were chopped off.
