Can I manage multiple Facebook pages under the same account?

Yes, you can add multiple Facebook pages (or multiple Instagram/Twitter/LinkedIn profiles, etc) to the one brand account and still have these under the same subscription.

The only time you would need to pay additional is if you wanted to create a totally separate brand in your SocialOmm account to manage each of your Facebook pages. You would do this if each of the pages were for separate businesses and you didn't want to mix your posts and scheduling calendar. You would also separate your Facebook pages into different brands if you have a separate team for each business that the Facebook pages are for. 

If you wanted each FB page to have its own brand account, then each brand would have its own subscription and all brands can be managed under the one SocialOmm account.