Unlike most abilities that use devotion to determine the magnitude of an effect, Mogis's Marauder uses devotion to determine the number of targets its ability has. Use your devotion to black as you put the ability on the stack. Mogis's Marauder's mana cost will always count toward your devotion to black. Once targets are chosen, it doesn't matter if your devotion to black changes while the ability is on the stack. The number of targets is locked in.

Numeric mana symbols ({0}, {1}, and so on) in mana costs of permanents you control don't count toward your devotion to any color.

Mana symbols in the text boxes of permanents you control don't count toward your devotion to any color.

Hybrid mana symbols, monocolored hybrid mana symbols, and Phyrexian mana symbols do count toward your devotion to their color(s).