First Sliver's Chosen — ruling 1

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.

First Sliver's Chosen — ruling 2

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.

First Sliver's Chosen : règles, fonctionnement et interactions (MTG)

First Sliver's Chosen is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

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.)

How it works : First Sliver's Chosen

Original English text — Wizards of the Coast / Scryfall

#1June 14, 2019· Wizards of the Coast
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.
No other card is named in this ruling.
#2June 14, 2019· Wizards of the Coast
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.
No other card is named in this ruling.