Retesting is testing this tast cases which failed and bugs were solved. Regression tests are test which check if added or changed functionality not caused faults in existing code. In your case retesting will be 50 in Module A and 40 in Module B. It is hard to say how many regression tests do you need but in such a situation (many test cases failed) probably you need to test all test cases. In general you do not test everything in regression.