Well, certainly not all of them are derived form the word "mercy". Even the ones in this database starting with "marc" don't come from it.
https://surnames.behindthename.com/names/start/marc There's for instance
Marchand which is from merchant. Or even
Marchesi "From the Italian title marchese meaning "marquis". It was probably a nickname for a person who behaved like a marquis or worked in the household of a marquis." And marquis ultimately comes from a Latin word meaning "frontier".
So whatever name you have in mind, it possibly may come from "mercy", I wouldn't know, but just because it starts with "marc" doesn't mean it does.
https://www.behindthename.com/pnl/109883