You may target zero, one, or two creatures. The creatures don't need to be untapped.

If you target zero creatures, Repel the Darkness can't be countered for having no legal targets. When it resolves, all that happens is that you draw a card.

If you target one creature and that target is illegal as Repel the Darkness resolves, the spell is countered. You don't draw a card.

If you target two creatures and they're both illegal as Repel the Darkness resolves, the spell is countered; you don't draw a card. If just one is illegal, the spell does resolve; the remaining legal target becomes tapped (if it's untapped at that time) and you draw a card.