around here, shrimp are sold/marketed by count per pound
the little ones = "rock shrimp" are 20+ per pound
I normally buy the 14-16 per pound size.
there are a lot of varieties marketed - but one needs a super dedicated source...
Florida Hoppers or Pink shrimp (Penaeus Duorarum)
Gulf Pink Shrimp (Litopanaeus duorarum, formerly Penaeus duorarum)
White Shrimp (Penaeus Setiferus)
Brown Shrimp (Penaeus Aztecus)
Rock Shrimp (Sicyonia Brevirostris)
Royal Reds (Pleoticus Robustus)
Northern Shrimp, Northern Prawn (Pandalus borealis)
Humpy Shrimp (Pandalus goniurus) - west coast
Gulf Brown Shrimp (Farfantepenaeus aztecus) - Gulf of Mexico
Gulf Pink Shrimp (Litopanaeus duorarum, formerly Penaeus duorarum)
Gulf White Shrimp (Litopanaeus setiferus)
Mexican Brown Shrimp (Litopanaeus californiensis, formerly Penaeus californiensis)
Northern Brown Shrimp (Farfantepenaeus aztecus)
Humpy Shrimp (Pandalus goniurus) - west coast
Coonstripe Shrimp (Pandalus hypsinotis) - Pacific
Sidestripe Shrimp (Pandalopsis dispar) - Pacific
Spot Prawns (Pandalus platyceros) - not true prawn
Chinese White Shrimp (Litopanaeus chinensis, formerly Penaeus chinensis, Penaeus orientalis)
Pacific White Shrimp (Litopenaeus stylirostris, formerly Panaeus stylirostris)