How to track the traffic of any website? Until unless the website owner publishes the data, it's not possible to know the visitor count they get. But many online tools out there which provide analysis and estimation of monthly traffic data. Not all the tools are so reliable and accurate but here are few of them which I oftenly use.
Best free tool for website traffic estimation.
Best free tool to track any website's traffic
Primarily SEMRush is search engine optimization tools. SEMRush Traffic Analytics is one of the tool under their umbrella which provides traffics estimation of any website. It provides Visitors count per month, Avg. visit duration, Bounce rate, and Country origin and other details.
Note: SEMRush is a freemium tool which has a limit for free users. Free accounts can make 10 total requests/day.
SEOquake is a browser extension for SEO from SEMRush. It provides on-page SEO audit, site ranking info, and many other stuff.
SEOquake also provides website traffic information. It uses API from SEMRush. Unless their website, this chrome extension seems don't have free user request limit. If you wanted to track your competitor's website traffic this is one best tool.