Once you have created a WordPress.com account, you can create as many websites with that account. Once you have multiple sites, clicking on “My Sites” at the top left of your screen will take you to a page where you will select the site you want to edit/manage. I believe there are no limits to the number of sites you can create.
Plans at WordPress.com are site-specific, not account-specific. So, when you create multiple sites, you will have to choose which plan you want for each of those sites. You could create multiple sites on the Free plan, or you could choose a different plan for each site. Each site will be billed individually, but you will be able to manage all your subscriptions from the WordPress.com account you have.
I hope this information helps. Let us know if you have any more questions. Cheers!