The Equipment that's unattached remains on the battlefield.

Akiri's first ability has you draw just one card per player you attack with an equipped creature, no matter how many equipped creatures you attack them with beyond the first and no matter how many Equipment that creature's carrying beyond the first.

If the creature is already tapped, it just gains indestructible until end of turn.

You choose which Equipment to unattach as Akiri's second ability resolves. Unattaching an Equipment from a creature won't affect other Equipment attached to that creature. No player can take actions between the time you unattach that Equipment and the time that the creature becomes tapped and gains indestructible.

Equipped creatures attacking a planeswalker or battle won't cause Akiri's first ability to trigger.