It depends. Like others stated, if doing a purpose specific spell, do not use the same candles for more than purpose. That’s obvious if you have already dressed the candle and carved it because once a candle is carved you can’t un-carve it and dedicate to another purpose. But even if you did not carve the candle or use any oils, if you did for example a love spell, for obvious reasons you would not want to use that same candle that has loving positive energy imprinted on it for a baneful curse, you could end up making your enemy obsess over you in an unhealthy way because you are mixing the two magick types or worse, make your enemy accidentally fall in love with you.
So for purpose specific spells and rituals, stick with one candle per each purpose. When doing evocation or maybe doing a ritual to call upon several entities just to commune with them, you can use the same candle unless of course, you have promised a particular entity that you would dedicate a certain candle for use with them only. But generally speaking, using the same candles for evocation to commune, or ask for advice, or share an offering or ask for a favor in general, is not a problem. It can actually make the candles stronger because each time you use them each individual spirit’s energy gets imprinted on everything in the room incuding the candle used to aid in calling it forth.
So the energy builds into one strong foundation. Sort of like how some people never clear their temple or working space of energy after their workings to let it build and create a strong magickal atmosphere within. But anytime you do a baneful working, you do want to cleanse your working space and if the candles are too large to be burnt all the way down with that first use, you can reuse them, just only reuse them when doing other baneful workings in the future, so set aside all candles used for baneful work that are not yet burnt out, by placing them in a box by themselves to contain that baneful energy together in one centralized dedicated location.