Q: Why Do We Light Two Candles (or More) on Shabbat?
A: Jews light a minimum of two candles on Shabbat, and not just one, to fulfill both versions of the fourth of the Ten Commandments in the Torah. The Ten Commandments appear twice in the Torah, with some variations in the language used, and therefore, carry additional meanings.
In the first version, in Exodus 20:8, the Fourth Commandment tells us to “zachor” or “remember the Sabbath to keep it holy.” In the second version, the commandment is to “shamor,” “to observe (or safeguard) the Sabbath to keep it holy.” Lighting two candles reminds us of the importance of attending to and taking action relating to both of these meanings when we welcome Shabbat.
Some Jews choose to light more than two Shabbat candles, most often lighting additional candles for each member of the family in the household. This is one way of carrying out the Jewish tradition of hiddur mitzvah, beautifying the mitzvah, with the goal of elevating our experience of lighting the Shabbat candles. Another is to use the most beautiful candles and candle holders we can.
Q: Why can’t we use two birthday candles instead of the traditional Shabbat candles?
A: Shabbat candles should be lit before the start of Shabbat and should continue burning for two to three hours once Shabbat has begun. It is Jewish tradition to light the Shabbat candles no later than 18 minutes before sunset. However, Reform Judaism is more flexible about this, and emphasizes the greater importance of lighting the Shabbat candles even if it is after sunset.
For further study: https://reformjudaism.org/shabbat-customs