As many as you like! If you’re an Editor, you can use the Mac app across multiple devices simply by signing in with your Sketch account.
However, please don’t share your account with other people. Each Editor in a Workspace can use their own Sketch account to log into the Sketch platform on any device they own – but sharing credentials to access and use Sketch is a violation of our terms of service agreement and may result in the suspension of your account.
In short: sharing sign-in details is a security risk and a terms of service violation. Don’t get your account suspended or deleted!