nefarox overlord of grixis double strike rules on ability?

[Teddy]

If I give Nefarox, Overlord of Grixis double strike and attack with him alone, does defending player sacrifice two creatures or just one?

[Kenji]

Just one. Nefarox's triggered ability has a trigger of "whenever Nefarox, Overlord of Grixis attacks alone." This trigger is met once, when you declare Nefarox as an attacker -- giving Nefarox double strike doesn't mean it was declared as an attacker a second time.