If you try to add additional contributors to your personal or team projects and get a message that you have reached the available limits for the number of contributors, this may mean:
You are on the Free plan: You have a project which has you and another user as contributors. In this case, you need to upgrade your plan to Startup or Business in order to be able to add more contributors.
You are on Pro, Startup, or Business plan: Your team has grown and the number of contributor seats purchased is already being used. In this case, you need to update your plan by adding additional seats.
You are on Pro plan (this plan was discontinued): Pro plan allows you to have max 5 contributors. If you already use all 5 seats on Pro plan and need to add more contributors, then you have to upgrade to the Business plan.