Text (FS+errata): 7/7. ; All Sliver creatures get +1/+1 for each other Sliver on the battlefield. [Oracle 2009/07/08]
This means that if there are 2 Slivers in play, each gets +1/+1. If there are 3, each gets +2/+2. If there are 4, each gets +3/+3. And so on. [D'Angelo 2007/05/15]