Say so that "I don't have a count of it". No body counts.
If you are expected to say some count, depending upon the
project duration and nature of product, you can give a
number. eg. around 400 for 1year project. you have to
sustain your answers because some have written 300 cases in
a day also.

factors are
1. need of the project(for simple functionalities many can
be written )
2.your role(a new tester may be assigned for execution than
creation)
................