As mentioned above, cherries have a tiny bioload. They are also prolific breeders and very good at hiding, given the right environment. I started with 6 (and a berried (pregnant) female and a bunch of juvies) in my 5 gal npt with my last boy; within 6 months I had about 20, then Alpha became blind and that population rose to too many to count (I could see more than 15 at any given time and more appeared during tank maintenance time). When I introduced Kappa, the number shrank significantly as his belly swole significantly. So how many you start with is up to you, how many you'll end up with is up to the betta and the shrimp.
If you want breeding (and keep in mind that your boy will see young cherries as a delicious snack, so each clutch will shrink a LOT from what you would expect to see in a predator-free environment) I would recommend getting 6-8 as the chances of getting males and females is greater. I special ordered mine at my LFS and the bag came with 6 adults, one of the females was berried, and about 10 juvies that were so tiny I doubt the shipper noticed when filling the bag (I only noticed them once I got them home!).