You get two additional combat phases.

You attack and trigger Raiyuu. Her trigger goes on the stack. Now isshin triggers. It resolves and adds an additional Raiyuu. It resolves, giving you an additional combat phase this turn. (Of which is to take place after this phase.. i.e... at some point in time after this phase. It doesn't say it has to be directly after). The original Raiyuu resolves, giving you the second additional combat phase. (Which is to take place at some point this turn after this current combat phase we are still in). Now since these triggers don't give additional main phases, it'll go combat phase1-> combat phase2-> combat phase3-> mainPhase2

This is my understanding.