You can't create separate workspaces from a single account. To create a new workspace for your client, they'd need to sign up with their own account.
However, you can add unlimited people to your existing workspace completely free - no extra charges for adding team members. [1] So if you want to collaborate with your client in the same workspace, you can just invite them to yours instead.
If you need any more help with managing workspaces or subscriptions, I’m here to assist. Would you like to share more about what you’re trying to achieve with your Clay account?