Each other creature you control enters the battlefield with a number of additional +1/+1 counters on it equal to Arwen, Weaver of Hope's toughness.
If Arwen, Weaver of Hope and another creature you control enter the battlefield at the same time, Arwen's ability won't cause that creature to enter with additional +1/+1 counters on it.
To determine how many additional +1/+1 counters a creature enters the battlefield with, use Arwen, Weaver of Hope's toughness as that creature is entering the battlefield.