According to the brochure linked above the number of still images that can be taken with the D850 on a single EN-EL15a or EN-EL15 battery has increased to 1840, compared to 1200 with the D810on a single EN-EL15 battery (all in the CIPA Standards test). That is a significant improvement.
The brochure also confirms that the D850 works with the current EN-EL15 batteries (as used in D800, D800e, D810, D500 etc), and not only with the EN-EL15a it comes with.