When a visitor hits your product page, we'll assign them a button which they will see for the remainder of their session - even if they navigate away from the page and come back. This assignment is random and distributed evenly amongst your site visitors.
You can have as many buttons in a campaign as you'd like and we will evenly spread them across your traffic.
If a visitor leaves your website for an extended period of time, then their session will likely expire. When this happens, they will be assigned a new button from your campaign the next time they visit your website.