You could certainly operate a few F-35Bs from Izumo although it would probably take some modifications. How many is a matter of the capacity to sustain operations - and as always, the next question is whether there are better ways to meet the need, or better ways to spend the money given 2 x B ≈ 3 x A.
I incline to the view that much of this is still in the realm of think-tank and defense-planner noodling combined with STOVL-enthusiast wishful thinking, amplified by the politically exciting idea of the JMSDF getting carriers and the predictable ROK #UsToo reaction thereto.