The number of domains you can enable SSL for in your Domain Manager depends on your Subscription level:
Basic subscription = 1 domain
Professional subscription = 4 domains
Enterprise subscription = 8 domains
Please Note: Shared SSL is included as part of the HubSpot Website add-on. Alternatively, dedicated SSL or custom third-party certificates can be purchased. If you're interested in these options, please contact your Customer Success Manager. You can also learn more here.