The question we get the most.
The simple answer is you need minimum 16Gb to operate Windows efficiently.
If you have 4 or 8GB then adding another 8GB is what you need. There is not much difference between 12GB and 16GB. An SSD would be a better buy if you have 12GB and wondering if you should buy another 8GB.
If you are using graphics program like Photoshop, Virtual Machines or gaming there is no limit to the RAM you system will use. Basically it will use everything you install. 32Gb is a good starting point.
If you have a 2015 to 2020 27 inch iMac then getting to 32GB is the best option.