Historical Rules · 13 November 2015
Rule 506.5
Official source text from the 13 November 2015 Rules release
A creature attacks alone if it’s the only creature declared as an attacker during the declare attackers step. A creature is attacking alone if it’s attacking but no other creatures are. A creature blocks alone if it’s the only creature declared as a blocker during the declare blockers step. A creature is blocking alone if it’s blocking but no other creatures are.