A standard 870 holds four rounds in the magazine tube. This has a "plus 2" extension added to the end of the tube, so that gives it a 6 round magazine tube capacity. You can add another round if you keep the chamber loaded as well.

6 in the tube, 1 in the chamber for a 6+1 capacity.

Hi Andre, with the 2+ extension, this has a capacity of 6+1, total 7rds.
Stay safe & have a great day!

Hi Andre and welcome. This is a 6 + 1 or 7rds total. Cheers and stay safe!