Is 2.5% (12.5k page views per day) good as compared to my total index pages which is 5,00,000 pages?
If not, how much % should be a good one?
It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, see the FAQ.
There is NO right or wrong answer. It is impossible to even guess without an idea on what your pages are about.
Think about it. Imagine if you have a news site with 5M articles, you probably have very few articles read each day since people reach much more about recent events. Then you are a store with 5M articles for sale and people only buy 2% of what you sell. You'd be out of business quickly!