Do not: fall for people who tell you to get tons of every size. Huge ones are expensive as hell and you probably won't have multiple battle-Cutters. You probably don't need many 1A or 2A or 3A prismatics either.
What you definitely need is:
- At least two Class 7 prismatics - these are used on Corvette and Conda.
- At least two Class 6 prismatics - these are used on FDS, FGS and Python. Although not very popular, some builds can take advantage of them.
- At least two Class 5 prismatics - these are used mainly on FDL, FAS and (in upcoming patch) Chieftain. Again, such builds aren't that popular (except probably FDL), but some people use them.
Class 4 and Class 3 prismatics can be used on trade ships in order to gain extra shield strength. Class 3 can also be used on Courier, but you may need a godroll G5 overcharged PP for it to function.
If you have a lot of cash (and you should), buy one Class 8. These are used on Cutter and T10.
Class 2 and Class 1 prismatics are generally worthless - seriously, who would want to put a prismatic shield on Sidewinder or Hauler?