Congrats on the babies! As Heather said, platys can store sperm, up to 7 months I believe, possibly longer.
Previous babies may have been eaten, or never born at all if the mother felt conditions were unsuitable, which I'm afraid may have been the case in your tank.
I don't know if you realise, but a 4 gallon tank is far too small for the fish you have to live out their natural lifespans. Kissing gouramis grow big (8 inches) and Chinese Algae eaters (if that is what you have) even bigger (10 inches); there is no way a 4 gallon can sustain them in the long run. Even a platy needs a bigger tank in order to display natural behaviour. You will need to do something about the overstocking if you want to see those babies grown into adults. There is also a risk in letting the CAE grow to adulthood in the same tank as the other lot, as they often grow aggressive and territorial in later life and can develop a taste for the slime coat of other fish.
While you are sorting out their housing situation, you need to work hard on keeping the babies' water clean by partial water changes. They need to be fed frequently (liquifry, or baby flakes, or in an emergency crushed ordinary flakes), and have plants to hide in from te other fish (including their mother).
