Women light 2 candles, 1 for “shomur” and one for ‘zochur”. There is a custom to add a candle for each child born, the basis for this is that if a woman missed lighting candles (which she would have done when hospitalized, or in the olden days, confined post-birth) there is a penalty imposed that has to light one extra from then on. Now, even when the woman does not miss lighting post-birth, she still adds one for her new baby.
Single girls/women living in their parents’ house generally don’t light (with the exception of chabad where girls light one), unless they are on their own and there is no one else to light in the house. The mitzva is that the candles need to be lit in the home, preferably by the woman of the house, but if she is not there, anyone can and must light.
One point, if you have consistently lit a certain number, it may be considered a neder (vow) for which you would need hatara nedarim (annuling a vow) to change the number.