how do I know how many visitors to the site? can I put google tags in the tags section?
You can edit your theme and use whatever analytics code you’d like in its layout.
Without editing the theme, there are also various contributed modules for adding analytics more easily. For example, there’s one for Google Analytics and one for Matomo/Piwik.
Furthermore, there is a module Statistics (Omeka S - Statistics) that do anaytics too. It can be used alongside with matomo. Remember that Google Analytics is not GRPD compliant (it steals a lot more data from users than needed), but Matomo is.
thank you, I will have a look 
You have to sign in on the Google analytics and link your site to see the traffic.