Can I host multiple websites within one Shared Hosting plan?

It varies on host to host, some web hosting providers allow you to host more than one website by adding secondary domains through cPanel's addon functionality. Whereas some web hosts allow you to host limited domains as per the plan you choose.

Yes, If you have selected a web hosting package with multiple/unlimited domains you can for sure do that. Also you can have subdomains under the same domain which work as a different site, if needed.

yes, you can host multiple websites with one shared hosting plan. it depends upon the plan which you have chosen.there is an option in the panel.

There is an option in cPanel, by using them you may easily do that. it depends on your hosting provider as well.

Yes, You can! Normally its depends on your hosting plans.