funny you found a keeper in the sensi stars... i once grew a pack, got lucky and got 9 females out of ten...and didn't get a keeper - but i just don't like that strain... i'm sure for a real indica-lover there would have been at least one keeper
but this question in general is very difficult to answer, some people open 100+ seeds when they select, some people just take clones from a random seedplant - it all comes down to what a keeper is for you... if you know what characteristics you are looking for you can start the research on which strains you want to grow - if you then select the strains carefully and grow at least 25 seeds i would be amazed if you can't find a plant that is good enough for you to keep it... but as always luck plays a big part as well - you can get a better plant growing one bagseed than starting with 100 seeds of a good and stable strain - if you are a real lucky (wo)man just keep on searching...and i'm sure you'll find your girl in the end... wish you better luck next time
but this question in general is very difficult to answer, some people open 100+ seeds when they select, some people just take clones from a random seedplant - it all comes down to what a keeper is for you... if you know what characteristics you are looking for you can start the research on which strains you want to grow - if you then select the strains carefully and grow at least 25 seeds i would be amazed if you can't find a plant that is good enough for you to keep it... but as always luck plays a big part as well - you can get a better plant growing one bagseed than starting with 100 seeds of a good and stable strain - if you are a real lucky (wo)man just keep on searching...and i'm sure you'll find your girl in the end... wish you better luck next time