Throttle Tabs by Eitan Isaacson

What does this extension do?

It allows you to limit your tabs to a set maximum. After the max number of tabs is reached, the least recently viewed tab is hidden in an overflow menu. The overflow menu can be throttled as well so that after a certain amount it will discard old tabs for good.

Why?

There are only that many tabs that a human can keep track of. Here is a blog post with some background.