It looks like using the class smite ability is a freebie after you hit somebody with a melee weapon attack. It says on page 107 of the Campaign Guide...
HC Class abilities wrote:
Starting at 2nd level, when you hit a creature with a melee weapon attack, you can expend one spell slot to deal radiant damage to the target, in addition to the weapon’s damage. The extra damage is 2d8 for a 1st level spell slot, plus 1d8 for each spell level higher than 1st, to a maximum of 5d8. The damage increases by 1d8 if the target is an undead or an infernal.
SO...
Question #1: Can you use this ability more than once per round? (if you have multiple attacks and score multiple hits)
Question #2: Does the class ability stack with the extra damage you would deliver if you had used your bonus action to cast Smite Heretic?
Thanks for any clarifications