Enter amounts shown in box 12 of your W-2 only in box 12 of TurboTax's W-2 form, nowhere else in TurboTax. Amounts shown in box 12 are not regular personal IRA contributions.
If you made actual regular personal traditional and Roth IRA contributions, not contributions to an employer's retirement plan, the combined total for someone age 50 or over in 2018 is the lesser of $6,500 or the compensation available to support the contribution.
(There is no kind of retirement account that has a 2018 contribution limit of $16,000. The 2019 elective deferral limit for someone age 50 or over to a SIMPLE IRA plan for 2019 is $16,000, but a SIMPLE IRA is contribution is reported with code S in box 12 of your W-2.)