I tend to agree with @Todd_Cavanaugh says.
It sounds like you have already created an Organisation. I think it is worth using this as it would make adding customers as guests a bit easier, and by default your guest clients would be mostly isolated from each other.
If you don’t have a domain, you could still achieve the same effect by creating a Workspace per customer, for example. It might start to get unwieldy as your customer base grows, but could work very practically for a small freelancer.
A disadvantage of workspaces is that you probably couldn’t create a task and copy to all your clients, for example monthly maintenance tasks that you’d do with all your customers. You probably can’t do searches and views across all your customers. There are 3rd party integrations you could use to overcome these problems.
An advantage is that each customer is totally isolated, and this might be mentally how you prefer to work (everything for one customer in one box, no distractions).
My discussion https://community.asana.com/discussion/201/should-i-use-workspaces-or-organisation might help
I’d suggest setting up 2 or 3 test emails and Asana logins, and use them to play with how Asana operates. You might find a function that works better one way and not the other for you and help you to decide by trial and error. As you are just starting up, switching styles wouldn’t bee too difficult.
Good luck @Sarah_Pluves