Free and Pro hold one workspace. Business holds three, and three is the most on any plan today. There is no extra-workspace add-on to buy, so needing more than your plan holds means moving up to Business. The plans are in Plans, credits and prices.
What you get
A workspace with its own Brain, voice, creator lists, Routines, weekly brief, chats and Collections. Fully separate from your others.
Credits are not separate. They belong to the account, so another workspace gives you somewhere else to work rather than more allowance to work with. Budget for both if you are taking on a client.
Adding one
New Workspace in the workspace menu at the top of the sidebar, or Settings, then Workspaces. You name it and go through a short brand setup.
If your plan's workspaces are all in use, that button offers the upgrade instead of creating one.
Do the brand setup properly before working in it. A client workspace with an empty brand writes generically, and that is more visible on client work than on your own.
Extras you already have
Some accounts hold more workspaces than their plan includes, from before this changed. Those stay open and keep working.
If your plan drops below what you are using, the extras are locked rather than deleted, and they open again when the plan covers them. See Your workspace is locked.
Removing one
Delete it in Settings, then Workspaces. Everything in that workspace goes with it permanently. The first workspace in the list has no delete button.
If a client relationship might resume, it is worth exporting anything you want to keep first.
When you do not need one
One brand across several platforms is one workspace. Platforms are handles inside Brain, not separate workspaces.
A one-off project is not a workspace either. Use a Collection and a few chats.
Reach for a workspace when the brand and voice are different, because that separation is what a workspace is for.