The more the better. lol. I go more by tank size to start off a group. a 30 or 40 gallon tank, only putting 5 shrimp in, the males may never find the female. a 10 gallon, you can start with 5. The best thing to do is get some, and then get some 6 months-year from a different breeder/supplier/source to introduce some new genes in now and again, but that's not needed as many people have had homogeneous breeding groups for a while without new genes, but you can risk the risk of genetic deformity. It's hard to say. A small group could breed for 5 years with no ill effects or you could start getting bad effects in 2 years or a year. There is no way to know for sure.
In reality, a lot of the shrimp we have all started with 1 or 2 mutations that popped up and were selectively bred from there. Wild Bee shrimp were breed to increase the black and white and created crystal blacks. Over huge breeding groups of those, a single crystal red appeared and that was selectively bred to breed out the red, so all crs are inbred really, as with dogs and many other creatures we breed for our needs. I can't remember the site, but it's out there that showed some of the balloon-head symptoms and other deformities to look for.
Whenever I try to selectively breed a female, I usually put a couple of males in a smaller tank or breeder box with a female. You don't want to put too many males or they may harass her to death trying to mate. 1 male may be sterile or not interested, so a couple of males usually are a good ratio.
20g platy, , 2 x 10g shrimp, 3 x 20g shrimp, 7.5g shrimp and 1 great dane/mastiff puppy.
Sump Pimp #2
My Tanks and my shrimps