Can I run a 12 2 wire off of a 20 amp breaker to a splitter box, then from the splitter box run one 12 2 wire to a light switch, then to 5 can lights? Then one more wire from the splitter box to another light switch connected to vent fan/light? Then run one more wire from the splitter box to a GFI outlet, then wire two more outlets from the GFI? Do I have to use GFI outlet after the first GFI? All this is off of one 20 amp breaker and will only be feeding the master bathroom.
Yes, you can do exactly what you propose. Just make sure the box you use is large enough for the splices.
You'll need at least a 4x4x1-1/2" box, which would be literally at full capacity. A 4x4x2-1/8" box would be a much better choice.
No, you do not need any GFI devices downstream of the first one, provided you wire the line in to the LINE terminals, and the line out to the other receptacles to the LOAD terminals.