While the testability of a requirement is fundamental to the concept of sizing with testable requirements, it should be noted that sizing a system using testable requirements is not the same as counting test cases.  The number of testable requirements may be very different from the number of test cases.  There are a number of reasons for this:

A testable requirement may require more than one test case to validate it.

Not all testable requirements will have associated test cases.  (Most systems test less than half the testable requirements.)

Some test cases may be designed to validate more than one testable requirement.