IMO get as much as you possibly can for an affordable price, possibly even populate them in a separate culture before adding them in. Make sure you have a humid area for the CUC, I overflow my water dish and have extra moss and stuff under there to keep it nice and moist for my CUC. First time I did bio they all died because I didn't have a good humid CUC spot for them
#How many springtails/powder oranges do I buy
keen sluice
Power oranges populate fast so if you start with your own separate culture you'll have lots to pop in. Plus if there's ever any die off in your tank you can top them up.