Usually the large bags contain 50pc or 100pc of fishes... those are the bulk bags for people who need to get larger group of the fishes at one go (for multiple or larger tank setups), in return they get to purchase them at a lower bulk price.
I have purchased bulk bags of tetras before to stock up tanks and the fishes were okay.
Whether the fishes are healthy will usually depend on the supplier and when the bags were delivered to the LFS, if it's only been a short while the fishes should still be healthy since the bag conditions have not hit toxic levels yet. Though since you can't pick and choose the fishes, the chances of getting a percentage of sick or weak fishes in the bag is also higher.
After all, those are the same fishes that are usually unpacked and put into retail tanks anyways. The difference is you can personally pick and choose those fishes, hence the higher prices when purchased individually.
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