There is no currently enforced limit to trusted testers. The way trusted testers get an update is just like any other update. Their profile checks in with the update server to see what the latest version available is. If it is greater than the one they have, they fetch it. The server can determine which version is the latest based on the user's profile - be it a trusted tester, standard user, percentage rollout cohort, etc.
Point being that if they are removed from trusted teters, they will continue to have the last version they received. It will remain on that version until their profile is given access to a version that is higher than the one they currently have - be it a standard release, another testing program, etc.
patrick