Aftermarket value priced replacement Rear Upper Control Arm (also referred to as upper rear wishbone).  Sold each.

Each MINI uses an upper and lower arm on each side (2-upper and 2-lower per car). The same part is used for right and left sides. Upper and Lower are not interchangeable. Upper and Lower control arms sold separately and individually.

Fits the following models:

2014-2021 F56 MINI Cooper and Cooper S Hardtop
2015-2021 F55 MINI Cooper and Cooper S Hardtop 4-Door
2016-2021 F57 MINI Cooper and Cooper S Convertible
2016-2021 F54 MINI Cooper and Cooper S Clubman
2017-2021 F60 MINI Cooper and Cooper S Countryman 

Also compatible with the F56E Cooper SE Electric Hardtop and F60H Cooper SE Countryman Hybrid.

The rear control arms affect rear toe and rear camber settings (among other things). Eccentric bolts provide minimal adjustments in the fractions of a degree. If greater adjustment is necessary such as when the car is lowered (inducing excessive negative rear camber), adjustable rear control arms are available for greater range of adjustment.

For the Powerflex Poly Bushings to fit the rear control arms, see Mini Cooper Rear Lower Lateral Arm Inner Bushing Powerflex Street Gen3 F56