Why does there need to be more than four RAM slots in a low-end consumer PC? As the consumers' need for RAM gets bigger the size range of the RAM sticks on the market gets bigger at the top end too, so there's already sufficient RAM capacity in four RAM slots for a budget PC. Let's say a consumer has a motherboard with 4 slots and each slot supports up to 16GB RAM. A typical consumer needs far less than 64GB RAM, so why should the consumer pay for more RAM than he needs?
In the case of medium- and high-end motherboards, some of these motherboards have 6 or more RAM slots. There's a theoretical limitation to the amount of RAM a motherboard can have, but this upper limit is far higher than a typical home PC user would ever encounter. The new Mac Pro model expected to be launched in 2022 will offer an option to be shipped with 128GB unified memory. Apple's new M1 Pro SoC places unified memory beside the other fundamental components, all on the same silicon. This results in faster memory bandwidth for both the CPU and the graphics card. source