Young mollies have fewer babies than older mollies.
Other fish in the tank will eat baby livebearers so if you have a male Betta, or any other fish in the tank, she might have had more but they got eaten.
Having lots of plants in the tank gives the babies somewhere to hide and there is less chance of them being eaten.
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If you have to add sodium bicarbonate (bicarb) to the tank to raise the pH, add some bicarb to a container of dechlorinated water and dissolve it completely. Then slowly pour some of that water into the tank near a filter outlet so it gets mixed in quickly.
A safer option than bicarb is to add some limestone, crushed coral or shells to the aquarium. These will help buffer the pH and stop it dropping and you won't have to add bicarb. Just add a small bit of limestone or shells and monitor the pH over a week. If it is stable at the pH you want, then don't add any more. If the pH is still dropping then add a bit more and monitor it over another week. Continue to add and monitor the limestone, shells or coral rubble until you get the desired pH.