For firms on the Team plan, you are currently allowed 1000 contacts. When you begin to approach this limit, a banner will be displayed on the Contacts page, warning you that you are approaching the limit and displaying how many remaining contacts you have.

Once you have reached the limit, users will no longer be able to add new contacts and a message will appear with instructions to upgrade or delete unused contacts. If you exceed the limit, you will retain access to all existing contacts and you will be able to edit and save them, you just cannot add more.