10
If you assign each bottle with a binary number between 1 and 999 (0000000001 and 1111100111), and assign 10 prisoners to a letter, you can make a sort of grid.
Each prisoner drinks from every bottle that has a 1 in their column.
When the prisoners die, you make a number with 1's for the prisoners that died, and 0's for the rest. That is your bottle.
Oh, and if none of them die, its the bottle that you didn't test.