How to use

TapVariableTargetCost

in

mage.abilities.costs.common

@Override
public TapVariableTargetCost copy() {
  return new TapVariableTargetCost(this);
}

public BurnAtTheStake(UUID ownerId, CardSetInfo setInfo) {
  super(ownerId,setInfo,new CardType[]{CardType.SORCERY},"{2}{R}{R}{R}");
  // As an additional cost to cast Burn at the Stake, tap any number of untapped creatures you control.
  this.getSpellAbility().addCost(new TapVariableTargetCost(filter, true, "any number of"));
  // Burn at the Stake deals damage to any target equal to three times the number of creatures tapped this way.
  this.getSpellAbility().addEffect(new BurnAtTheStakeEffect());
  this.getSpellAbility().addTarget(new TargetAnyTarget());
}