The promise is for one every 15 minutes from each terminus. Keolis Amey/TfW say:
We will invest £738m to transform the valley lines to Treherbert, Aberdare, Merthyr Tydfil, Rhymney and Coryton, electrifying 172 km of track and upgrading infrastructure to enable improved journey times and more trains every hour.
Remove Pacer trains by December 2019.
Introduce a Central Metro that improves journey times and increases frequency to at least four trains per hour from the head of each using new trains.
Introduce new Metro Vehicles with level boarding by December 2022, which will provide a modern metro-style service to the Treherbert, Aberdare and Merthyr valleys.