I wrote a Chrome extension to limit the amount of tabs in Google Chrome. It's called 'Nuff tabs. There were already some extensions in the wild that do something similar, but none with the properties I was looking for (lightweight, specific options, etc.).
It works as follows. In the options you can specify the maximum number of open tabs. When you open one more tab, the extension will automatically close a previous tab. In the options you can also specify which rule it must follow to determine which tab to close: either close the oldest tab, the least recently used tab, the least frequently used tab, or a random tab. My current favorite setting is the "least recently used" tab, as this way I don't have to worry about losing older tabs that are still relevant.
You can also choose to display the number of open tabs in the extension icon.
Install it and let me know what you think.
Chrome store link: https://chrome.google.com/webstore/detail/nuff-tabs/kemeihccgedidlokcbfhdekcfojpjjmp
Source code: https://github.com/steven2358/NuffTabs