In NC my ex and I have years, 2020 I claim my child. We have 50/50 custody. How do I determine whole year or not

Unless you and your spouse file a joint tax return, a child can only be a claimed as a dependent by one parent.

The one who had custody for more than 1/2 of the year can claim the child as a dependent, child care expenses, earned income tax credit and, if eligible, Head of Household.

The custodial parent can transfer the exemption to the non-custodial parent by providing them with a signed copy of Form 8332.  It is their choice to do so.   If required to do so by a divorce decree or child custody agreement, and the child's other parent does not follow the agreement, your recourse is in the family court, as the IRS will not attempt to enforce such agreements.