They have short lifespans (1 year average) so if you want them around, have the guy/gal @ the LFS catch you the smallest ones. Ghost shrimp also molt as they grow, shed their exoskeleton (think of snakes), they are at their most vulnerable during this time and are probably the primary cause for some of their deaths.
In planted tanks they hard to spot. I *should* have 10 or more shrimps but I only see 3 come to the front during feeding, so who knows? The smallest shrimp I purchased 3+ months ago whom I thought would not survive is still doing quite well and is spotted daily during food drops.
Here is one of mine chilling...obvious female, green gunk at her abdomen are eggs.