website splits - sample sizes

Id like to do some research into website splits.

As visitors arrive to one of our websites (lets call it id=123), the page will randomly deliever one of three pages.

The goal is for that page to then push the user to our ordering page. The research project aims to identify which of the three pages is most effective and to tweak the page.

What sample size would you need for each of the randomly delievered pages to ensure you have statistically significant data?