That depends on which extension you get, in most cases with 2¾" shells a +2 will bring you up to 8+1 or a plus 3 will be 9+1. The reason I say most cases is some 2¾" shells have a different style of Crimp on the end, and you can actually lose one round from the total capacity l, you also lose capacity if you load 3" shells.
A couple of +2, and +3 extensions available here
gggaz.com/870-mag-extension.html

Joe, +2 as it will be slightly longer than the barrel with over a 2 round extension. Cheers and stay safe!