It's really very simple and logical.
The PIC 10 has 6 pins, and the PIC 12 has 8 pins. The PIC 16 uses the 14 bit instruction set, except when it uses the 12 bit instruction set, or when the model number is 4 digits starting with 1, then it uses the enhanced 14 bit instruction set. The PIC 18 uses the 16 bit instruction set, and the dsPIC 30 the 24 bit instruction set. The PIC 14 and 17 never existed (shhhh, don't tell anyone). Now we have the PIC 24 and 32 which break the pattern, so you'll just have to remember those separately. Oh, and there's a 33, but that's just a 30 that runs on 3.3 V.
See, simple, right?
