The slogan the more memory the better seems to have become the catch phrase as the price of memory has dropped and programs have become more memory hungry. But, even if your mother board has many slots, additions of memory have to be made in accordance with the manual that comes with the motherboard. One cannot just add memory until all the slots are filled nor, as a rule, can one mix different types of memory.
Memory is inserted into banks on the motherboard.
The combinations each bank can hold are part of the configuration of the motherboard. For example, on my motherboard there are three banks - bank 0, bank 1, and bank 2. Only certain combinations of memory can go in those banks. I can only put a pair of 8 megs simms. On the other hand, I could put 2-64 meg simms in bank 0, etc.