"Mac" and "Mac + iOS" subscription members can use Setapp only on one Mac.
If you need Setapp on more Macs at the same time, you can upgrade to the "Two Macs" plan (2 Macs) or to the "Power User" plan (4 Macs and 4 iOS devices). The "Manage devices" page of your online Setapp account is a place to manage the computers you're using Setapp on. For tips on how to upgrade, see this article.
However, you can transfer your subscription even if you have a single device in your account:
Click the Setapp icon in the menu bar, then click your profile picture and choose Sign Out.
Sign in to Setapp on another computer using your credentials.
Alternatively, follow these steps: