All Sliver creatures get +1/+1 for each other Sliver on the battlefield.
5/1/2007 If Sliver Legion and two other Slivers are on the battlefield, for example, each of them gets +2/+2.
7/1/2013 Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like flying, having more than one instance of the ability doesn’t provide any additional benefit.
7/1/2013 If the creature type of a Sliver changes so it’s no longer a Sliver, it will no longer be affected by its own ability. Its ability will continue to affect other Sliver creatures.