There really is no magic number in regard to the amount of fluids you need. The best guideline is the amount that keeps your urine output plentiful and dilute. So, basically, that means that if you are voiding 3-5 times a day, are not thirsty or feeling lightheaded, you are probably OK. If you are sweating or in a warm environment, you need extra fluids (everyone does!).
Understand though that caffeine and sugary drinks (even Gatorade) can increase your stool output (and caffeine will artificially increase your urine output). So, it can be a challenge, especially early on, when you are learning and your body is adapting.
Your mom just is being concerned, and rightly so, since hydration seems to be one of our biggest challenges. Another tip, don'y keep your stomach full of fluids, but sip a little all day long. That way you are more likely to absorb it all.
Jan