In the background, bench start initiates a simple werkzeug server for your default site. If you have other sites on the same host, they will not be served.
I suspect you could run the command more than once in different terminal sessions to serve different sites, but I’ve never tried it. Generally speaking, bench start is intended only for development. For multiple tenants, you would usually set up a production deployment (which includes nginx and supervisorctl).
Edited: Looks like my understanding was incorrect! See @revant_one’s correct answer below
bench start will serve all sites as long as currentsite.txt is not configured.
I create sites like: site1.localhost, site2.localhost, siteN.localhost, I can access them via hostname like: http://siteN.localhost:8000, http://site2.localhost:8000