Self Explanatory(?)
Swords Dance raises the user's Attack by two stages.
Source
This means your attack is doubled. Every stage adds .5x to the multiplier. (One stage is 1.5x, two stages is 2x, etc.)
The exact numbers for each stage: -6: .25x -5: ~.29 -4: .33x -3: .4x -2: .5x -1: .67x 0: 1x 1: 1.5x 2: 2x 3: 2.5x 4: 3x 5: 3.5x 6: 4x