That will all depend on what you have purchased.
A SAP will typically come with 1 IP Address. You can send emails from a SAP domain (being it's an authenticated domain)
You can additionally buy/register multiple private domains on the same business unit and again send emails from those private domains. These private domains can send from the same IP Address.
You can only have 1 SAP for a business unit, but you can have multiple private domains.
Equally, you can multiple IP addresses for a business unit. Sending from an IP address will depend on many factors i.e. volumes, reputation etc. Typically you want to dedicate an IP address for transactional sends and the you could share you other IPs across the other sub domains.
You could alternatively dedicate an IP address per private domain, but it is a wider architectural decision and based on your brand, volumes and practice.