An easy way to remember is more ends with the er sound like greater and most ends with st just like greatest. (it's caused a few spelling mistakes for me in the past, but it helped me to remember … To use the correct adjective with the phrase in detail, think about fewer vs less in number vs amount - but remember in detail means specifically or completely already.

What's more is an expression that's used when you want to emphasize that the next action or fact is more or as important as the one mentioned. War doesn't bring peace; What's more, it brings more …