Is the CFM56-7B26 is flat rated to 26K?
 

Question :

I heard that cfm56-7b is a flat rated engine. So is the cfm56-7b26 engine is flat rated to 26K?

The CFM56-7B engine is defined as a flat rated engine up to the flat rating take-off limit of ISA +15c (30c at sea level).

In fact, you'll find that most (maybe all?) modern engines are flat rated.

And ISA+15 is a very common limit for the flat rating, although there are exceptions. Check performance data for the aircraft/engine of interest.