The rule of thumb is to use at least one 120mm radiator (section) per each water cooled component plus one additional section.
For example, if you're liquid-cooling a CPU and a single high-performance graphics card, we recommend using at least one 360mm (3x 120mm) radiator for the best results. More radiators will give you more cooling power. For example, if you use Ryzen 5950X and Nvidia 3090, we recommend two radiators for optimal cooling. Motherboard and memory water blocks usually have lower power output, and therefore they are not included in this equation.
Radiators also come in different sizes, but the 120mm type is the most common. For example, a 280mm radiator (2x 140mm), built for 140mm cooling fans, has a 33% larger cooling surface than the 240mm variant (2x 120mm), meaning it can dissipate substantially more heat given the same conditions. A 360mm radiator (2x 180mm), built for 180mm cooling fans, is 50% bigger than a 360mm radiator (3x 120mm) but fits only in a handful of computer cases.
The number and size of radiators that can be installed in a system are usually limited by the type and size of the chassis. That is why EK recommends the use of larger, water-cooling friendly computer chassis.