A Performance-optimized CU can serve 5 million 128-dimensional vectors.
A Capacity-optimized CU can fit 25 million 128-dimensional vectors.
A Cost-optimized CU can serve 25 million 128-dimensional vectors or 5 million 768-dimensional vectors.
Since your collection's schema may differ from the ones in the simple guide above, we highly recommend you test the actual requirements against different CU types.