Yea, there is nothing built-in that would automatically limit 1 at a time.
What you could do is require Approvals to elevate and then it would be on the approvers to check to see if somone is already elevated and reject the PIM request:
Otherwise you would have to spin up something on your own to check that automatically or add and remove users from the PIM group dynamically. Sounds like more work that its worth though.
Be sure to have global admin breakglass accounts as well so you dont lock yourself out!