Welcome Sher.
I find that if I want my livebearer fry to survive I need to start with the tougher fish like mollies or even goodeids. They have larger fry that are better developed at birth than swordtails, platies or guppies. With their bigger size, they are less likely to fall to predation by the parent fish in the first few minutes of their lives. I always use a separate birth tank of at least 10 gallons when I want a high fry survival rate.
Several other factors help quite a bit. One is the cover that the fry have to escape predation even by the less eager fry eaters. Cover is best done with fine leaved plants in dense clumps. Plants that I find suitable are the mosses.
Water is also a consideration. I find that a hard water high pH tank is ideal for most common livebearers and even some of the less common ones. The right water makes for healthier parents and thus healthier fry. After the fry drop, it is usually best to remove the female from the birthing tank so that she will not eat any of the fry although there are some livebearers where removal of the parents is not needed.
Rapid fry growth is best achieved by using a mix of several different foods including both live foods, frozen foods, flake foods and even vegetable based foods. Because the heavy feeding needed to assure good growth will end up fouling the water, lots of large partial water changes are also needed.
With decent care of fry in suitable water with good feeding and frequent water changes, I usually see at least 80% survival to adulthood. There are times that the rate is much higher but I don't ever count on much more. After all, even in a well maintained tank of adult fish, there are sometimes unexplained losses.
Even common aquarium mollies can produce lots of long lasting fry. I kept a female in a tank with her fry for the time between fry drops so that I could document both the fry growth and the parent's next pregnancy so I took lots of pictures. This was a very peaceful molly who rarely chased her fry so they were fairly safe with her. At about 5 weeks, this is how my 10 gallon looked with all the fry still surviving. I never did see one of them dead but there may have been minor losses that I just did not see.
Edit to add comment and picture.