First Sliver's Chosen

Sliver creatures you control have exalted. (Whenever a creature you control attacks alone, it gets +1/+1 until end of turn for each instance of exalted among permanents you control.)

Jumpstart: Historic Horizons

The most tenacious members of the hive instinctually guard the First Sliver.

Modern Horizons

A creature attacks alone if it’s the only creature declared as an attacker during the declare attackers step (including creatures controlled by your teammates, if applicable). For example, exalted won’t trigger if you attack with multiple creatures and all but one of them are removed from combat.

Whenever a creature you control attacks alone, count the number of instances of exalted among permanents you control. After those abilities resolve, that’s how many times the creature will get +1/+1.