The Samsung Galaxy S9 and S9+ will keep the same battery capacities as their predecessors, with the smaller sibling to house a 3000mAh battery, and the plus-sized variant to opt for a 3500mAh one.
https://www.notebookcheck.net/The-Samsung-Galaxy-S9-will-ship-with-a-3000mAh-battery-go-on-sale-on-March-16.278670.0.html