ideally, just 1, but that's more of a lottery. it is my opinion that if you have to legit go through 100 seeds to find 1 keeper, that is not a very impressive strain. at ALL.
a 10 pack should be more than enough to find a keeper if the strain is legit. mr. nice comes in packs of 18, and in my experience, most of them are keepers.
i lucked out on a pineapple chunk freebie 2 years ago. i got the magic pheno but there's a bunch of people shitting on the strain because they grew several and never found this pheno. i'd hesitate to buy a pack based on this but when you do find a "keeper" it's usually worth it. not 100 seeds, though. that's nuts.