Sajica are inept predators at best, and not piscivores. This doesn't mean they won't eat live bearer fry, or any fish that gets caught up in a precarious spot.
The way many dithers avoid predation is in a shoal or school that is constantly moving, the activity tends to confuse predators. So as far as how many to get, get a large enough shoal that provides that type confusion will help. I usually find shoals of 6-10 are a good start, as long as the tank can tolerate the extra bioload.
I would also remove obviously pregnant females to drop fry in a safe tank or sump, to add more numbers to the shoal as time goes by. Live bearers are not generally long lived fish, so getting 2 years out of an individual is a pretty good run.