Teachers can only be enrolled in 50 active classes at one time. This includes classes where they are the primary of the second teacher.
There is no limit to the number of archived classes a teacher can be enrolled in.
If you import/join more than 50 classes from Google Classroom or Clever, you might not see them all on your Manage Classes or Assignments page, even if you were able to successfully import them. Email email@example.com for further assistance.