How much attic/ceiling insulation does the energy code require by climate zone?

Ceilings need R-30 in zone 1, R-38 in zones 2-3, and R-49 in zones 4-8 (IRC Table N1102.1.1).

The IRC energy-efficiency table sets minimum ceiling (attic) insulation by climate zone: R-30 in zone 1, R-38 in climate zones 2 and 3, and R-49 in zones 4 through 8. These are the prescriptive minimums for the building thermal envelope.

To actually achieve the rated R-value over the exterior walls, the insulation has to keep its full thickness above the wall top plate, which is why raised-heel (energy) trusses are commonly used at the eaves so the attic insulation isn't compressed.

This is Table N1102.1.1 in the 2012 IRC (renumbered Table N1102.1.2 in 2015); the R-30/38/49 values are the same in both editions. Your local jurisdiction may amend the adopted edition, so confirm which one applies.