There are several ways to gauge how much traffic your competitors are getting:
- Google Trends will tell you how many times their brand name was organically searched. This is free.
- Similarweb and Alexa estimate, not only traffic, but demographic, traffic sources and share of pageviews. Note that some data is paid. Alexa is by far the cheaper option.
- Ahrefs has a solid paid solution as well as a trial.