Hi @soapy ,
Each CU is a full installation of Exchange that includes updates and changes from all previous CUs so you can go directly from CU12 to CU19. And as suggested by AshokM, considering that you have already installed .Net 4.8, it's recommended to upgrade to CU19 right away.
Additionally, please be sure to install the KB5000871 right after upgrading to CU19 in order to fix the known security vulnerabilities including March 2021.
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