Ahmet,
This R2, in C4D R20 shows 4 GPU tick boxes, BUT does not accept them ticked. Therefore leaves only 2 GPUS on my 4 GPU system.
Tried latest 441 Studio Nvidia drivers, the same issue..
Fact 1: Octane log within C4D shows 4GPU.
Fact 2: R19 - Shows 4 GPU / 4 activated
Fact 3: Standalone 2019.1.4. shows 4 and all 4 are working.
my only solution was to downgrade the plugin to 2019.1.4