Roughly speaking yes, 2 licences per team of two people. And one more thing. Even if your developer has his personal licence, when you add him to the team, you have to pay for it again.
So it’s easier if you add him to the team as a paid member right away.
You pay for a place in the team. And if your developer will work with different clients, in different teams, he will have to be paid for again.
Or another option. You have separate subscriptions. And you send the developer a file which he will later add to his team folder and can work separately in every sense. Because you lose the ability to see all the changes live. You can only see, but not work with the project.