if you have not done curved piecing yet an apple core is a good beginning- because it is not a 'severe' curve. give them a try-
as for how many----it certainly depends on how large your pieces are-and how big you want your quilt to be--if you look at the table runner above- it has 40 apple cores-
it is a good (on going-scrappy) project---cut a bunch- sew them together---cut a bunch more- instead of trying to cut a (million) all at once
some projects it is easier-and makes more sense to cut as you need to-just start with an assortment- put them together---also-if you have not done these before- it would be better to start with (maybe 40 like above) and put them together---if you decide you hate sewing them you have not commited yourself--and are not (stuck) with a million pieces you may never use- better to start with a few-that you could make into a tablerunner- or a placemat- or tote bag---instead of cutting enough for a king sized quilt then hating the process!