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.
2020-04-17 • wotc
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.
2020-04-17 • wotc
The target tapped creature will be exiled even if the creature you tapped leaves the battlefield or becomes untapped before Swallow Whole resolves.
2020-04-17 • wotc
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.
2020-04-17 • wotc
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.