That wouldn’t really be a term limit so much as a fixed term, then—and for partial/suffect terms, it might conflict with the original intention that the Supreme Court be immune from short-term political intrigues. (And of course it would also give some presidents more than two appointments per term.)
Another option might be to appoint one justice strictly every two years, with no fixed term and no fixed number of justices. Then the number of justices would fluctuate around half the number of years in the average term—so nine justices with an average term of 18 years, twelve justices with an average term of 24 years, etc.
That wouldn’t really be a term limit so much as a fixed term, then—and for partial/suffect terms, it might conflict with the original intention that the Supreme Court be immune from short-term political intrigues. (And of course it would also give some presidents more than two appointments per term.)
Another option might be to appoint one justice strictly every two years, with no fixed term and no fixed number of justices. Then the number of justices would fluctuate around half the number of years in the average term—so nine justices with an average term of 18 years, twelve justices with an average term of 24 years, etc.