You must tap exactly one creature to cast Swallow Whole; you can’t cast it without tapping a creature, and you can’t tap additional creatures.

If an effect copies Swallow Whole, the copy will put a +1/+1 counter on the creature tapped to pay the additional cost of the original spell.

If the target creature is an illegal target by the time Swallow Whole tries to resolve, the spell won’t resolve. You won’t put a +1/+1 counter on any creature.

You can tap any untapped creature you control, including a creature you haven’t controlled continuously since the beginning of your most recent turn, to pay the additional cost.

The target tapped creature will be exiled even if the creature you tapped leaves the battlefield or becomes untapped before Swallow Whole resolves.