Under ideal conditions female guppies can produce a batch of young every month. Young females will produce smaller batches of young more often, whereas the older females produce bigger batches less often.
A big healthy female can have 100 or more young in each batch but more commonly they only produce 20 or 30 fry (fry = baby fish).
Female livebearers, (Guppies, Mollies, Swordtails, etc) can carry sperm packets from previous matings. The females use these sperm packets to fertilise successive batches of eggs inside their body. This means a female can get pregnant without a male being present in the tank.
Young guppies can breed when about 3 months old. However, it is best to allow the females a chance to get to full size before they breed. This means you have to separate the males from females as soon as you can sex them.
Male guppies get the big colourful tail and usually have more colour on their body. They also have a modified anal (bottom) fin that is long and pointed. The female’s anal fin is fan or triangular shaped.